Hydrogen absorbing alloy, method of surface modification of the alloy, negative electrode for battery and alkaline secondary battery

ABSTRACT

A hydrogen-absorbing alloy which is excellent in stability in an aqueous solution and in mechanical pulverizability is disclosed. This hydrogen-absorbing alloy contains an alloy represented by the following general formula (I): 
     
       
         Mg 2 M1 y   (I) 
       
     
     wherein M1 is at least one element selected (excluding Mg, elements which are capable of causing an exothermic reaction with hydrogen, Al and B) from elements which are incapable of causing an exothermic reaction with hydrogen; and y is defined as 1&lt;y≦1.5.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is division of U.S. patent application Ser. No.08/787,101, filed Jan. 22, 1997 which is a continuation-in-part of U.S.patent application Ser. No. 08/505,154, filed Jul. 21, 1995, nowabandoned.

BACKGROUND OF THE INVENTION

This invention relates to a hydrogen-absorbing alloy, a method ofmodifying the surface of the hydrogen-absorbing alloy, negativeelectrode for battery and an alkaline secondary battery.

Hydrogen-absorbing alloy is known as being capable of stably absorbingand storing hydrogen several ten thousands times (calculated as a gasunder normal temperature and pressure) as much as of its own volume.Therefore, hydrogen-absorbing alloy is noticed as a promising materialfor safely and easily storing, keeping and transporting hydrogen as anenergy source. Hydrogen-absorbing alloy is also studied for utilizationin a chemical heat pump or compressor by making most of a difference inproperty between hydrogen-absorbing alloys, some of them being developedfor actual use. Recently, the application of hydrogen-absorbing alloysto a metal hydride secondary battery (for example, nickel-hydrogensecondary battery) as an energy source by making use of hydrogen storedin a hydrogen-absorbing alloy, as well as an electrode material bymaking use of its high catalytic activity in the absorption anddesorption reaction of the hydrogen-absorbing alloys has beenextensively developed.

As evident from these facts, the hydrogen-absorbing alloy has manypossibilities for various applications in view of its physical andchemical characteristics, so that the hydrogen-absorbing alloy is nowconsidered as being one of important raw materials in future industrial.

The metal capable of absorbing hydrogen and constituting thehydrogen-absorbing alloy may be in the form of single substance whichreacts exothermically with hydrogen, i.e., a metal element capable offorming a stable compound together with hydrogen (for example, platinumgroup elements, lanthanum group elements and alkaline earth elements);or in the form of an alloy comprising such a metal, as mentioned above,alloyed with another kind of metals. One of the advantages of the alloyresides in that the bonding strength between a metal and hydrogen can besuitably weakened so that not only the absorption reaction but also thedesorption reaction can be performed comparatively easily. Secondadvantage of the alloy resides in that the absorption and desorptioncharacteristics of the alloy with respect to the magnitude of hydrogengas pressure required for the reaction (equilibrium pressure; plateaupressure), the extent of equilibrium region (plateau region), the change(flatness) of equilibrium pressure during the process of absorbinghydrogen and the like can be improved. Third advantage of the alloyresides in the improvement in chemical and physical stability.

The composition of the conventional hydrogen-absorbing alloy may beclassified into the following types; i.e., (1) an AB₅ type (for example,LaNi₅, CaNi₅); (2) an AB₂ type (for example, MgZn₂, ZrNi₂); (3) an ABtype (for example, TiNi, TiFe); (4) an A₂B type (for example, Mg₂Ni,Ca₂Fe); and other types (for example, cluster), wherein A represents ametal element which is capable of exothermically reacting with hydrogen,and B, another kind of metal. Among them, LaNi₅ of (1), a laves phasealloy belonging to (2) and some kinds of alloy belonging to (3) arecapable of reacting with hydrogen at the normal temperature, andchemically stable so that they are extensively studied as a candidatefor an electrode material of a secondary battery.

Whereas, the hydrogen-absorbing alloy belonging to (4) A₂B type isaccompanied with the following problems. Namely, the alloy stronglyattract hydrogen so that hydrogen once absorbed therein can be hardlyreleased. The absorption and desorption reaction thereof occurs onlywhen the temperature thereof is raised up to a relatively high degree(about 200 to 300° C.), and the rate of the reaction, if occurred, isslow. The chemical stability, in particular the stability in an aqueoussolution, of the alloy is comparatively low. The alloy is generally veryviscous and hard so that the working such as pulverization of it is verydifficult. In view of these facts, the hydrogen-absorbing alloy of A₂Btype is rarely utilized except for the storage and transport of hydrogenin spite of its excellent capacity of absorbing hydrogen which iscomparable to other types of hydrogen-absorbing alloy on the basis ofvolume and, if calculated on the basis of weight, two to several timesas high as that of other types of hydrogen-absorbing alloy. Therefore,of these problems inherent to the hydrogen-absorbing alloy of A₂B typeas explained above are solved, it would be possible to expand theapplication of the alloy not only to the same fields as those of othertypes of hydrogen-absorbing alloy but also to a new field ofutilization.

By the way, there have been reported a number of academic papers on thehydrogen-absorbing alloy of this (5) type. However, up to date, thereport of practical use or testing for practical use is almost none.

Meanwhile, there is disclosed in Jpn. Pat. KOKAI Publication No. 6-76817a magnesium-based hydrogen-absorbing alloy represented by a compositionformula of Mg_(2−x)Ni_(1−y)A_(y)B_(x) (wherein x is 0.1 to 1.5; y is 0.1to 0.5; A represents an element selected from Sn, Sb and Bi; Brepresents an element selected from Li, Na, K and Al) such for exampleas Mg_(1.5)Al_(0.5)Ni_(0.7)Sn_(0.3); orMg_(1.8)Al_(0.2)Ni_(0.8)Sn_(0.2). There is also disclosed in thispublication that the hydrogen-absorbing alloy can be utilized as anegative electrode material of an alkali secondary battery. However,since this hydrogen-absorbing alloy disclosed in the publication isfundamentally of A₂B type, the hydrogen-absorbing and desorbing propertythereof in the normal temperature region is poor. Therefore, in order tomake it possible to absorb and desorb hydrogen under normal temperatureand pressure, the hydrogen-absorbing alloy is covered on the surfacethereof with a nickel metal compound or a phosphorous compound asdisclosed in the publication.

As explained above, the A₂B type hydrogen-absorbing alloy has a featuredistinct from other types of hydrogen-absorbing alloy in that it islight in weight, large in capacity and low in raw material cost sinceits composition is mainly consisted of alkaline earth metals and irongroup elements. However, the A₂B type hydrogen-absorbing alloy isaccompanied with various problems as explained above.

BRIEF SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide ahydrogen-absorbing alloy which is chemically stable, in particular in anaqueous solution, and can be easily pulverized by way of mechanicalmeans.

Another object of this invention is to provide a hydrogen-absorbingalloy which is improved of its hydrogen-absorbing property, inparticular the hydrogen-absorbing at room temperature.

Another object of this invention is to provide a method of modifying thesurface activity of the hydrogen-absorbing alloy so that hydrogen can beeasily and sufficiently absorbed by the hydrogen-absorbing alloy.

A further object of this invention is to provide a negative electrodefor a secondary battery, which is excellent in stability during theelectrode reaction, and to provide an alkali secondary battery having animproved charge/discharge cycle property.

A still further object of this invention is to develop a means toevaluate the deteriorating rate of a hydrogen-absorbing alloy containingmagnesium, and, on the basis of this means, to provide a negativeelectrode suited for practical use, which is excellent in thereversibility and stability in the electrode reaction and an alkalisecondary battery provided with such a negative electrode.

Namely, according to the present invention, there is provided ahydrogen-absorbing alloy containing an alloy represented by thefollowing general formula (I):

Mg₂M1_(y)  (I)

wherein M1 is at least one element selected (excluding Mg, elementswhich are capable of causing an exothermic reaction with hydrogen, Aland B) from elements which are incapable of causing an exothermicreaction with hydrogen; and y is defined as 1<y≦0.5.

According to the present invention, there is further provided ahydrogen-absorbing alloy containing an alloy represented by thefollowing general formula (II):

Mg_(2−x)M2_(x)M1_(y)  (II)

wherein M2 is at least one element selected (excluding Mg) from thegroup consisting of elements which are capable of causing an exothermicreaction with hydrogen, Al and B; M1 is at least one element selected(excluding Mg and M2) from elements which are incapable of causing anexothermic reaction with hydrogen; x is defined as 0<x≦1.0; and y isdefined as 1<y≦2.5.

Further, according to the present invention, there is also provided ahydrogen-absorbing alloy containing an alloy represented by thefollowing general formula (III):

M_(2−x)M2_(x)M1_(y)  (III)

wherein M is at least one element selected Be, Ca, Sr, Ba, Y, Ra, La,Ce, Pr, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu, Ti, Zr, Hf, Pd and Pt;M2 is at least one element selected (excluding M) from the groupconsisting of elements which are capable of causing an exothermicreaction with hydrogen, Al and B; M1 is at least one element selected(excluding Mg and M2) from elements which are incapable of causing anexothermic reaction with hydrogen; x is defined as 0.01<x≦1.0; and y isdefined as 0.5<y≦1.5.

Moreover, according to the present invention, there is further provideda method of modifying the surface of a hydrogen-absorbing alloycomprises a step of treating the hydrogen-absorbing alloy with an R—Xcompound, wherein R represents alkyl, alkenyl, alkynyl, aryl or asubstituted group thereof; X is a halogen element.

Still more, according to the present invention, there is furtherprovided a hydrogen-absorbing alloy, wherein a half-width Δ(2θ) of atleast one peak out of peaks of three strongest lines to be obtained byan X-ray diffraction of the alloy using Cukα-ray as a radiation sourcelies in the range of 0.2°≦Δ(2θ)≦50°.

Still more, according to the present invention, there is furtherprovided a hydrogen-absorbing alloy which comprises 10% or more ofmagnesium, wherein an apparent half-width Δ(2θ₁) of a peak in thevicinity of 20° lines in the range of 0.3°≦Δ(2θ₁)10°, or an apparenthalf-width Δ(2θ₂) of a peak in the vicinity of 40° lies in the range of0.3°≦Δ(2θ₂)≦10° in an X-ray diffraction of the alloy using CuKα-ray as aradiation source.

Moreover, according to the present invention, there is further provideda method of modifying the surface of a hydrogen-absorbing alloycomprising a step of mechanically treating the hydrogen-absorbing alloyunder vacuum or in an atmosphere of an inert gas or hydrogen.

Moreover, according to the present invention, there is further provideda method of modifying the surface of a hydrogen-absorbing alloy, whichcomprising the steps of mechanically treating the hydrogen-absorbingalloy under vacuum or in an atmosphere of an inert gas or hydrogen.

Furthermore, according to the present invention, there is provide anegative electrode for battery containing a hydrogen-absorbing alloycomprising an alloy represented by the following general formula (I):

Mg₂M1_(y)  (I)

wherein M1 is at least one element selected (excluding Mg, elementswhich are capable of causing an exothermic reaction with hydrogen, Aland B) from elements which are incapable of causing an exothermicreaction with hydrogen; and y is defined as 1<y≦1.5.

According to the present invention, there is further provided an alkalisecondary battery comprising a negative electrode containing ahydrogen-absorbing alloy comprising an alloy represented by thefollowing general formula (I):

Mg₂M1_(y)  (I)

wherein M1 is at least one element selected (excluding Mg, elementswhich are capable of causing an exothermic reaction with hydrogen, Aland B) from elements which are incapable of causing an exothermicreaction with hydrogen; and y is defined as 1<y≦1.5.

According to the present invention, there is provided a negativeelectrode for battery containing a hydrogen-absorbing alloy comprisingan alloy represented by the following general formula (II):

Mg_(2−x)M2_(x)M1_(y)  (II)

wherein M2 is at least one element selected (excluding Mg) from thegroup consisting of elements which are capable of causing an exothermicreaction with hydrogen, Al and B; M1 is at least one element selected(excluding Mg and M2) from elements which are incapable of causing anexothermic reaction with hydrogen; x is defined as 0<x≦1.0; and y isdefined as 1<y≦2.5.

According to the present invention, there is further provided an alkalisecondary battery comprising a negative electrode containing ahydrogen-absorbing alloy comprising an alloy represented by thefollowing general formula (II):

Mg_(2−x)M2_(x)M1_(y)  (II)

wherein M2 is at least one element selected (excluding Mg) from thegroup consisting of elements which are capable of causing an exothermicreaction with hydrogen, Al and B; M1 is at least one element selected(excluding Mg and M2) from elements which are incapable of causing anexothermic reaction with hydrogen; x is defined as 0<x≦1.0; and y isdefined as 1<y≦2.5.

According to the present invention, there is provided a negativeelectrode for battery containing a hydrogen-absorbing alloy, wherein ahalf-width Δ(2θ) of a least one peak out of peaks of three strongestlines to be obtained by an X-ray diffraction using Cukα-ray as aradiation source lies in the range of 0.2°≦Δ(2θ)≦50°.

According to the present invention, there is further provided an alkalisecondary battery comprising a negative electrode containing ahydrogen-absorbing alloy, wherein a half-width Δ(2θ) of a least one peakout of peaks of three strongest lines to be obtained by an X-raydiffraction using Cukα-ray as a radiation source lies in the range of0.2°≦Δ(2θ)≦50°.

According to the present invention, there is provided a negativeelectrode for battery containing a hydrogen-absorbing alloy comprisingmagnesium, wherein, when the negative electrode is immersed in a 6N to8N aqueous solution of an alkali hydroxide, (a) either the elution rateof magnesium ion into the aqueous solution of alkali hydroxide of normaltemperature is not more than 0.5 mg/kg alloy/hr, or the elution rate ofmagnesium ion into the aqueous solution of alkali hydroxide of 60° C. isnot more than 4 mg/kg alloy/hr, and (b) either the elution rate ofcomponent element of alloy into the aqueous solution of alkali hydroxideof normal temperature is not more than 1.5 mg/kg alloy/hr, or theelution rate of a component element of alloy into the aqueous solutionof alkali hydroxide of 60° C. is not more than 20 mg/kg alloy/hr.

According to the present invention, there is further provided an alkalisecondary battery comprising a negative electrode accommodated in a caseand containing a hydrogen-absorbing alloy comprising magnesium, apositive electrode accommodated in the case and so arranged s toopposite the negative electrode with a separator sandwichedtherebetween, and an alkali electrolyte filled therein;

wherein a magnesium ion concentration in the alkali electrolyte 30 daysor more after filling and sealing the alkali electrolyte in the case isnot more than 2.2 mg/liter.

According to the present invention, there is further provided ahydrogen-absorbing alloy containing an alloy represented by thefollowing general formula (V):

(Mg_(1−x)M3_(x))_(20−y)M4  (V)

wherein M4 is at least one element selected Ni, Fe, Co, Cu, Zn, Sn andSi; M3 is at least one element selected (excluding the elements of M4)from the group consisting of elements which are more electronegativethan Mg; x is defined as 0<x<0.5; and y is defined as 0≦y<18.

Further, according to the present invention, there is also provided ahydrogen-absorbing alloy containing an alloy represented by thefollowing general formula (VI):

(Mg_(1−x)M5_(x))_(20−y)M6  (VI)

wherein M5 is at least one element (excluding elements which are moreelectronegative than Mg) which has an atomic radius 1 to 1.5 times ashigh as that of Mg; M6 is at least one element selected Ni, Fe, Co, Cu,Zn, Sn and Si; x is defined as 0<x<0.5; and y is defined as 0≦y<18.

Moreover, according to the present invention, there is further provideda hydrogen-absorbing alloy which is formed of a mixture comprising:

an alloy having hydrogen-absorbing properties; and

at least one additive selected from the group consisting of (a) at leastone element selected from Group IA elements, Group IIA elements, GroupIIIA elements, Group IVA elements, VA elements, Group VIA elements,Group VIIA elements, Group VIIIA elements, Group IB elements, Group IIBelements, Group IIIB elements, Group IVB elements, Group VB elements andGroup VIB elements; (b) an alloy formed of any combination of elementsdefined in the (a); and (c) an oxide of any of elements defined in the(a);

the mixture being mechanically treated under vacuum or in an atmosphereof an inert gas or hydrogen.

Moreover, according to the present invention, there is further provideda hydrogen-absorbing alloy, which comprises;

an alloy having hydrogen-absorbing properties; and

0.01 to 50% by volume of at least one powdered additive having 0.01 to100 μm in average diameter, which is dispersed in the alloy and selectedfrom the group consisting of (a) at least one element selected fromGroup IA elements, Group IIA elements, Group IIIA elements, Group IVAelements, VA elements, Group VIA elements, Group VIIA elements, GroupVIIIA elements, Group IB elements, Group IIB elements, Group IIIBelements, Group IVB elements, Group VB elements and Group VIB elements;(b) an alloy formed of any combination of elements defined in the (a);and (c) an oxide of any of elements defined in the (a).

According to the present invention, there is further provided an alkalisecondary battery comprising a negative electrode containing ahydrogen-absorbing alloy comprising an alloy represented by thefollowing general formula (V):

(Mg_(1−x)M3_(x))_(20−y)M4  (V)

wherein M4 is at least one element selected Ni, Fe, Co, Cu, Zn, Sn andSi; M3 is at least one element selected (excluding the elements of M4)from the group consisting of elements which are more electronegativethan Mg; x is defined as 0<x<0.5; and y is defined as 0≦y<18.

Further, according to the present invention, there is also provided analkali secondary battery comprising a negative electrode containing ahydrogen-absorbing alloy comprising an alloy represented by thefollowing general formula (VI):

(Mg_(1−x)M5_(x))_(20−y)M6  (V)

wherein M5 is at least one element (excluding elements which are moreelectronegative than Mg) which has an atomic radius 1 to 1.5 times ashigh as that of Mg; M6 is at least one element selected Ni, Fe, Co, Cu,Zn, Sn and Si; x is defined as 0<x<0.5; and y is defined as 0≦y<18.

Moreover, according to the present invention, there is further providedan alkali secondary battery comprising a negative electrode containing ahydrogen-absorbing alloy which is formed of a mixture comprising:

an alloy having hydrogen-absorbing properties; and

at least one additive selected from the group consisting of (a) at leastone element selected from Group IA elements, Group IIA elements, GroupIIIA elements, Group IVA elements, VA elements, Group VIA elements,Group VIIA elements, Group VIIIA elements, Group IB elements, Group IIBelements, Group IIIB elements, Group IVB elements, Group VB elements andGroup VIB elements; (b) an alloy formed of any combination of elementsdefined in the (a); and (c) an oxide of any of elements defined in the(a);

the mixture being mechanically treated under vacuum or in an atmosphereof an inert gas or hydrogen.

Moreover, according to the present invention, there is further providedan alkali secondary battery comprising a negative electrode containing ahydrogen-absorbing alloy, the hydrogen-absorbing alloy comprising:

an alloy having hydrogen-absorbing properties; and

0.01 to 50% by volume of at least one powdered additive 0.01 to 100 μmin average diameter, which is dispersed in the alloy and selected fromthe group consisting of (a) at least one element selected from Group IAelements, Group IIA elements, Group IIIA elements, Group IVA elements,VA elements, Group VIA elements, Group VIIA elements, Group VIIIAelements, Group IB elements, Group IIB elements, Group IIIB elements,Group IVB elements, Group VB elements and Group VIB elements; (b) analloy formed of any combination of elements defined in the (a); and (c)an oxide of any of elements defined in the (a).

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention and, together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a graph showing a phase diagram of Mg/Ni-based alloy;

FIG. 2 is a graph showing a phase diagram of La/Ni-based alloy;

FIG. 3 is a partially sectioned perspective view showing a cylindricalsecondary battery according to one embodiment of the present invention;

FIG. 4 is a graph showing the relationship between the value of y inMg₂Ni_(y) and the amount of magnesium eluted;

FIG. 5 is a partially sectioned perspective view illustrating a methodof measuring a maximum stress of a band-like piece of an alloy;

FIG. 6 is a block diagram illustrating a hydrogen-absorption anddesorption performance evaluation apparatus of temperature-scanning typeto be employed in the examples of this invention;

FIG. 7 is a graph showing the relationship between the rise intemperature of hydrogen-absorbing alloys of Examples 16 and 17, as wellas of Comparative Examples 3 and 11 and the changes in pressure (adecrease in pressure in proportion to the hydrogen absorption by ahydrogen-absorbing alloy);

FIG. 8 is a graph showing the relationship between the rise intemperature of hydrogen-absorbing alloys of Example 18 and ComparativeExample 3 and the changes in pressure (a decrease in pressure inproportion to the hydrogen absorption by a hydrogen-absorbing alloy);

FIG. 9 is a graph illustrating the changes in pressure during hydrogenabsorption at a temperature of 25° C. as obtained by using a Mg₂Nihydrogen-absorbing alloy which has not been surface-modified and anotherMg₂Ni hydrogen-absorbing alloy which has been surface-modified;

FIG. 10 is a graph showing the relationship between the number of cyclesin a simulated cell which has been provided with either a negativeelectrode of Example 151 or a negative electrode of Comparative Example20;

FIG. 11 is a graph showing the relationship between the number of cyclesand the discharge capacity in a simulated cell which has been providedwith either a negative electrode of Example 242 or a negative electrodeof Comparative Example 20; and

FIG. 12 is a graph showing the XRD patterns of hydrogen-absorbing alloyswhich have been obtained by mechanically treating the mixturescomprising Mg₂Ni and Ni.

DETAILED DESCRIPTION OF THE INVENTION

A Hydrogen-absorbing alloy according to an embodiment of this inventioncontains an alloy represented by the following formula (I):

Mg₂M1_(y)  (I)

wherein M1 is at least one element selected (excluding Mg, elementswhich are capable of causing an exothermic reaction with hydrogen, Aland B) from elements which are incapable of causing an exothermicreaction with hydrogen; and y is defined as 1<y≦1.5.

Examples of elements (M1) are Fe, Ni, Co, Ag, Cd, Mn, In, Se, Sn, Ge andPb. This M1 may be used as a single element or as a mixture comprisingtwo or more of these elements. Preferable examples of the M1 are thosehaving a higher electronegativity than that of Mg, i.e., Fe, Ni, Co, Ag,Cd, Mn, In, Se, Sn, Ge and Pb. In particular, a hydrogen-absorbing alloycontaining iron group elements such as Fe, Ni, Co as a alloying elementis preferable, since such an alloy is chemically stable and excellent inhydrogen-absorbing and desorbing performance. M1 is preferably anelement which is more electronegative than Mg and which provides, ifused in 10 atomic % or less based on pure magnesium, an alloy havingless crystal lattice of Mg_(1−w)M1_(w) phase (0<w≦0.1), in volume, thancrystal lattice of pure magnesium. Examples of this element are Mn, Ag,Cd and In.

Table 1 shows the volume of crystal lattice of each of an alloyconsisting of the substituting element as mentioned above and magnesium,and pure magnesium.

The values indicated in this Table 1 are calculated on the basis of thelattice constants obtained from the diffraction patterns of the alloysas measured according to the powder X-ray diffraction method, assumingthat the crystal structures of these alloys are formed of the hexagonalsystem lime that of pure magnesium.

By the way, if the value of W in the Mg_(1−w)M1_(w) phase exceeds over0.1, the crystal structure of the Mg_(1−w)M1_(w) phase may takedifferent crystal structure from the hexagonal system so that thechanges in magnesium crystal lattice volume due to the addition of theelement M1 may not be accurately evaluated. Therefore, the range of Wwas limited to 0<w≦0.1. However, if it is clear that the crystalstructure of the Mg_(1−w)M1_(w) phase can be maintained in the form ofhexagonal system even if the value of W exceeds over 0.1, the element M1may be optionally selected from a range wherein W exceeds over 0.1.

TABLE 1 crystal Lattice Volume Composition of alloy (nm³) Mg (Pure Mg)0.0462 Mg_(0.99)Ag_(0.01) 0.0459 Mg_(0.9)Cd_(0.1) 0.0452Mg_(0.95)In_(0.05) 0.0460

The reasons for limiting the range of y of M1 in the general formula (I)to 1<y≦1.5 will be explained below in relative to (a) Influence to theAmount of Hydrogen Absorption; and (b) Influence to Chemical Stabilityand Workability such as pulverizability.

(a) Influence to the Amount of Hydrogen Absorption

If the ratio between Mg having a hydrogen-absorption capacity and M1(which is hardly capable of forming a hydrogenated compound) is set to2:y, the value of y should be y=1 (namely, Mg₂M1) in stoichiometric viewpoint. Practically, however, if the value of y is set to 1 or less,problems may be caused with respect to the chemical stability andmechanical pulverizability of the resultant alloy, which are the subjectmatters this invention is intended to solve. On the contrary, if valueof y is excessively large, other several problems may be raised therebymaking it undesirable in practical use. For example, if value of y ismore than 2, the resultant alloy is, microscopically speaking, no moreformed only of Mg₂M1 type-based crystal, but is turned into a MgM1₂type, i.e., Laves phase. This MgM1₂ type alloy also has ahydrogen-absorbing capacity. However, the hydrogen-absorbing capacity ofMgM1₂ type alloy is only 40 to 70% of that of Mg₂M1 type alloy on thebasis of weight. Therefore, an excessive value of y would bedisadvantageous in terms of capacity density.

(b) Influence to Chemical Stability and Pulverizability

The Mg₂M1 type hydrogen-absorbing alloy is generally formed only whenthe condition of Mg:M1=2:1 is met. Therefore, it is almost impossible tomaintain the Mg₂M1 type structure, if some compositional fluctuation,such as shortage or excess of Mg or M1 exists locally in the alloy. Themechanism of this will be explained with reference to a phase diagram asfollows. FIG. 1 shows a phase diagram of Mg₂Ni (Mg—Ni system)representing a typical example of Mg₂1 alloy. FIG. 2 shows a phasediagram of LaNi₅ (La—Ni system) of AB₅ type. These phase diagrams aredisclosed in “Binary Alloy Phase Diagrams” ASM International (USA),1990. As seen from these FIGS. 1 and 2, Mg₂Ni is indicated by a singlevertical Line in the phase diagram of Mg—Ni system, whereas LaNi₅ isindicated as somewhat expanded region in the phase diagram of La—Nisystem. This can be attributed to the facts in the process ofmanufacturing an alloy that in the case of LaNi₅, even if thecomposition of melt is fluctuated more or less from the prescribedcomposition, an alloy which is substantially the same as LaNi₅ can beproduced. But in the case of Mg₂Ni, if the composition of melt ispluctuated from the prescribed composition, it will cause theco-precipitation of Mg₂Ni and an excessive component in a state of2-phase eutectoid according to the fluctuation of the melt.

Namely, in a case where y is expressed as a positive number in anequation of Mg:Ni=2:y, if Y<1, Mg₂Ni and an excessive Mg will beco-precipitated as a eutectoid, whereas if Y>1, a eutectoid of Mg₂Ni andMg₂Ni₂ or a eutectoid of Mg₂Ni, Mg₂Ni₂ and Ni will be formed.

Mg is inferior in chemical stability such as corrosion resistance andoxidation resistance, and is higher in viscosity and malleability ascompared with Ni. On the other hand, when Mg₂Ni is compared with Mg₂Ni₂,Mg₂Ni is more resistive to water and oxygen, since Mg₂Ni is constructedto be higher in polarization (ionicity) in the stated of hydrogenabsorption in particular. Therefore, an alloy containing y falling in acondition of y<1 is poor in chemical stability and in pulverizabilitythough it can withstand against a mechanical stress owing to thepresence in the grain boundary of highly viscous Mg. By contrast, wheny>1, there is no co-precipitation of Mg, and moreover the alloy isconstructed such that the surface of Mg₂Ni is enclosed by Mg₂Ni or Ni,whereby improving the chemical stability thereof. Moreover, when y>1,the resultant alloy is highly rigid, but susceptible to brittle fractureat the grain boundary phase containing an excessive amount of Ni of lowviscosity, thereby making it possible to be easily pulverized bymechanical means.

As explained above, one of the conditions for realizing the improvementof mechanical pulverizability is y>1, when the ratio of Mg and M1components are expressed as Mg:M1=2:y. Meanwhile, in view of maintaininga sufficient capacity of the alloy without relying on the MgM1₂ phase,the upper limit of y should be set to 1.5. Further, in view of chemicalstability of the alloy, the range of y should be set to 1<y≦1.5.

When the ratio of Mg and M1 components are expressed as Mg:M1=2:y, itwill be theoretically sufficient if the lower limit of y exceeds over 1.However, as a matter of fact, there are a fluctuation in composition andsegregation in the alloy so that one of the conditions to obtain analloy wherein every portions within the alloy are occupied by acomposition whose value of y exceeds over 1 is dependent on theuniformity of the alloy. Specifically, (a) The value of y shouldpreferably be 1.05 or more, if the alloy is to be manufactured by theso-called annealing method wherein a melt of component elements preparedusing an induction furnace or an arc furnace in the same manner as inpreparing an ordinary metal ingot is poured into the vessel such as amold; (b) The value of y should preferably be 1.02 or more, if the alloyis to be manufactured by allowing the melt mentioned above to becontacted with a low temperature/high heat capacity material such as arotating roll or a liquid thereby quenching the melt, or by allowing themelt mentioned above to be injected into air or liquid thereby quenchingthe melt; and (c) The value of y should preferably be 1.02 or more, ifthe alloy is to be manufactured by mixing several kinds of pure metalsor alloys so as to formulate a predetermined alloy composition and thensubjecting the mixture to a hot rolling, a hot press or a mechanicalmixing (a mechanical alloying method) without subjecting the mixture toa melting process.

According to the method of (a), the segregation is more likely to becaused during the slow cooling step thereby making it more difficult toobtain a homogeneous alloy as compared with other methods. However,since the manufacturing process is rather simple, the method of (a) ismost extensively utilized. According to the method of (c), theuniformity of alloy is more likely to be influenced by the manufacturingconditions, so that, depending on the manufacturing conditions, thelower limit of y may be required to be raised. By contrast, according tothe method of (b), an alloy of comparatively homogeneous quality can beobtained. If the homogeneity of composition and texture can be enhancedby the optimization of manufacturing conditions or by a treatment suchas annealing after manufacturing, it is possible to achieve the objectof this invention even with a composition containing 1.01 or more of y.The judgment of the homogeneity can be performed by varioussurface-analyzing methods (such as EDX; energy dispersed X-rayspectrometer or EPMA; electron probe microanalyzer) using an electronmicroscope or by an X-ray diffraction method. For example, according tothe surface-analyzing method, an alloy is determined as beinghomogeneous when 90% or more of a sectioned surface is made up of thesame phase in the measurement of component distribution in a sectionedtexture of an alloy. On the other hand, according to the X-raydiffraction method, the ratio between the magnitude of a diffractionpeak belonging to a mother alloy such as Mg or M1, or single elementsincluded in the mother alloy and the magnitude of a diffraction peak tobe derived from these elements when they are existed as an individualsubstance is represented by a percentage, and the resultant values aretotaled, thus determining the homogeneity of an alloy if the total isnot more than 5%.

In view of these results, it is expected that the lower limit of y is inthe range of about 1.01 to 1.10 even in an alloy to be manufactured byother manufacturing methods.

The hydrogen-absorbing alloy according to this invention may be an alloyrepresented by the general formula (I) containing Group VB elements orGroup VIB elements up to 20 atomic %.

As explained above, the hydrogen-absorbing alloy according to thisinvention includes an alloy represented by the general formula (I):Mg₂M1_(y) (wherein M1 is at least one element selected (excluding Mg,elements which are capable of causing an exothermic reaction withhydrogen, Al and B) from elements which are incapable of causing anexothermic reaction with hydrogen; and y is defined as 1<y≦1.5). Namely,since the hydrogen-absorbing alloy represented by the general formula(I) is featured in that y or M1 such as Ni is more than 1 and less than1.5, it is chemically stable and excellent in mechanicalpulverizability, exhibiting a high hydrogen-absorbing capacity which isinherent to the A₂B type alloy such as Mg₂Ni.

Therefore, the hydrogen-absorbing alloy according to this invention iscapable of maintaining its excellent hydrogen-absorbing capacity even ifit is allowed to react with a hydrogen gas containing a small amount ofan oxidizing gas such as oxygen or water vapor. Still more, even if thehydrogen-absorbing alloy according to this invention is caused tocontact with an aqueous solution, the hydrogen-absorbing capacitythereof would hardly be deteriorated, thus extending the utility of thealloy.

Further, a hydrogen-absorbing alloy in general is gradually refined as aresult of the expansion and contraction of crystal lattice due to theabsorption and desorption of hydrogen, thereby causing change inphysical properties (such as bulk density, contact resistance andconductivity). If this change in physical properties causes a trouble,it can be avoided by adopting a method of using an alloy powder whichhas been pulverized in advance. Since the hydrogen-absorbing alloyproposed by this invention can be more easily pulverized as comparedwith the conventional A₂B type alloy such as Mg₂Ni, the problemmentioned above can be easily dealt with.

As explained above, the hydrogen-absorbing alloy represented by thegeneral formula (I) according to this invention can be easily andreliably treated in a preliminary working before use, and the controlthereof during use can be easily and reliably performed. Therefore, thehydrogen-absorbing alloy represented by the general formula (I) is quiteuseful as an electrode material of a secondary battery.

A hydrogen-absorbing alloy according to another embodiment of thisinvention contains an alloy represented by the following formula (II):

Mg_(2−x)M2_(x)M1_(y)  (II)

wherein M2 is at least one element selected (excluding Mg) from thegroup consisting of elements which are capable of causing an exothermicreaction with hydrogen, Al and B; M1 is at least one element selected(excluding Mg and M2) from elements which are incapable of causing anexothermic reaction with hydrogen; x is defined as 0<x≦1.0; and y isdefined as 1<y≦2.5.

As examples of M1, the same elements as explained with reference to thegeneral formula (I) may be employed.

Examples of the elements (excluding Mg) which are capable of causing anexothermic reaction with hydrogen, or capable of spontaneously forming ahydrogenated compound are alkaline earth elements such as Be, Ca and Ba;rare earth elements such as Y, Ra, La, Ce, Pr, Pm, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm and Lu; Group IVa elements such as Ti, Zr and Hf; and GroupVIIIa elements such as Pd and Pt. This M2 may be used as a singleelement or as a mixture comprising two or more of these elements.

M2 should preferably be selected (excluding Mg) from Al, B and theelements which are capable of causing an exothermic reaction withhydrogen and have a higher electronegativity than that of Mg. Namely, M2should preferably be at least one element selected from B, Be, Y, Pd,Ti, Zr, Hf, Th, V, Nb, Ta, Pa and Al. When an element having a higherelectronegativity than that of Mg is selected as M2, it is possible tominimize that difference in electronegativity between the alloy andhydrogen and to instabilize hydrogen within the lattice thereby toimprove the hydrogen-absorbing property of the alloy. In particular, Bewhich is an alkaline earth metal is capable of forming a chemicallystable alloy when it is alloyed with Mg. On the other hand, Ti, Zr andHf belonging to Group IVa elements are highly reactive to hydrogenthereby forming a hydrogenated compound.

Additionally, M2 should preferably be selected (excluding Mg) from Al, Band the elements which are capable of causing an exothermic reactionwith hydrogen, which provides, if used in 10 atomic % or less based onpure magnesium, an alloy having less crystal lattice of Mg_(1−w)M1_(w)phase (0<w≦0.1), in volume, than crystal lattice of pure magnesium.Namely, M2 should preferably be at least one element selected from Liand Al.

The reason for limiting the range of x in the general formula (II) isbased on the following facts. Namely, when the value of x exceeds over1.0, the hydrogen-absorbing properties of Mg₂M1_(y) (hydrogen-absorbingcapacity, flatness of plateau region and reversibility) are badlyaffected, and, in some cases, the crystal structure per se can not bemaintained. Therefore, the range of x should preferably be 0.05≦x≦0.5.

The reason for limiting the range of y in the general formula (II) isbased on the following facts. Namely, the advantages that can beobtained by setting the value of y to more than 1 are the same asalready explained above in reference to the alloy of the general formula(I). On the other hand, when the value of y exceeds over 2.5, not onlythe hydrogen-absorbing capacity of the alloy is reduced, but also thecrystal structure inherent to the alloy may not be maintained.Therefore, the range of y should preferably be 1.01≦y≦1.5, morepreferably 1.02≦y≦1.5, and more preferably 1.05≦y≦1.5.

This hydrogen-absorbing alloy according to this invention may be analloy represented by the general formula (II) containing Group VBelements or Group VIB elements up to 20 atomic %.

As explained above, the hydrogen-absorbing alloy. according to thisinvention includes an alloy represented by the general formula (II):Mg_(2−x)M2_(x)M1_(y) (wherein M2 is at least one element selected(excluding Mg) form the group consisting of elements which are capableof causing an exothermic reaction with hydrogen, Al and B; M2 is atleast one element selected (excluding Mg and M2) from elements which areincapable of causing an exothermic reaction with hydrogen; x is definedas 0<x≦1.0; and y is defined as 1<y<2.5). Namely, since thehydrogen-absorbing alloy represented by the general formula (II) ischaracterized in that part of Mg is substituted with an elementrepresented by M2 such as Al, the hydrogen-absorbing property, inparticular the lowering of hydrogen-absorbing temperature can beimproved as compared with the conventional A₂B type hydrogen-absorbingalloy, and at the same time a high hydrogen-absorbing capacity inherentto the A₂B type Hydrogen-absorbing alloy can be maintained. Further,this hydrogen-absorbing alloy represented by the general formula (II) islarger in hydrogen-absorbing capacity (based on weight), lower inmanufacturing cost and lighter in weight as compared with theconventional rare earth-based hydrogen-absorbing alloy. Moreover, sincethe hydrogen-absorbing alloy represented by the general formula (II) ischaracterized in that y of M1 such as Ni is more than 1 and less than2.5, it is chemically stable and excellent in mechanicalpulverizability.

Therefore, the hydrogen-absorbing alloy according to this invention iscapable of maintaining its excellent hydrogen-absorbing capacity even ifit is allowed to react with a hydrogen gas containing a small amount ofan oxidizing gas such as oxygen or water vapor. Still more, even if thehydrogen-absorbing alloy according to this invention is caused tocontact with an aqueous solution, the hydrogen-absorbing capacitythereof would hardly be deteriorated, thus extending the utility of thealloy.

As explained above, the hydrogen-absorbing alloy represented by thegeneral formula (II) according to this invention is capable of loweringhydrogen-absorbing temperature and at the same time capable ofsustaining a high hydrogen-absorbing capacity inherent to the A₂B typehydrogen-absorbing alloy. Moreover, this alloy represented by generalformula (II) can be easily and reliably treated in a preliminary workingbefore use or in the control of conditioning during use thereof.Therefore, the hydrogen-absorbing alloy represented by the generalformula (II) is quite useful as an electrode material of a secondarybattery.

A hydrogen-absorbing alloy according to another embodiment of thisinvention contains an alloy represented by the following formula (III):

M_(2−x)M2_(x)M1_(y)  (III)

wherein M is at least one element selected Be, Ca, Sr, Ba, Y, Ra, La,Ce, Pr, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu, Ti, Zr, Hf, Pd and Pt;M2 is at least one element selected (excluding M) from the groupconsisting of elements which are capable of causing an exothermicreaction with hydrogen, Al and B; M1 is at least one element selected(excluding Mg and M2) from elements which are incapable of causing anexothermic reaction with hydrogen; x is defined as 0.01<x≦1.0; and y isdefined as 0.5<y≦1.5.

As examples of M1, the same elements as explained with reference to thehydrogen-absorbing alloy containing an alloy represented by the generalformula (I) may be employed.

As examples of M2, the same elements as explained with reference to thehydrogen-absorbing alloy containing an alloy represented by the generalformula (II) may be employed.

Preferable examples of the combination of M, M2 and M3 are a ternaryalloy comprising Zr as M, Fe as M1 and Cr as M2; and a quarternary alloycomprising Zr as M, Ni and Co as M1 and V as M2.

The reason for limiting the ranges of y and x in the general formula(III) is based on the following facts. Namely, if the value of y is lessthan 0.5, the individual phases of M, M1 and M2 will be precipitated sothat the features inherent to the hydrogen-absorbing alloy would belost, and at the same time the hydrogen-absorbing alloy would becomechemically unstable. Therefore, it is preferable to set the lower limitof y to over 1.0 (for example, 1.01). On the other hand, when the valueof y exceeds over 2.0, not only the hydrogen-absorbing capacity of thealloy is reduced, but also the crystal structure inherent to the alloymay not be retained. Therefore, the upper limit of y should preferablybe set to 1.5.

If the value of x is less than 0.01, a hydrogen-absorbing alloy havingan excellent hydrogen-absorbing property at a low temperature would nomore be obtainable. On the other hand, when the value of x exceeds over1.0, not only the crystal structure of the hydrogen-absorbing alloy isaltered, but also the properties inherent to the A₂B type alloy may belost. Therefore, a preferable range of x is 0.05 to 0.5.

As explained above, the hydrogen-absorbing alloy according to thisinvention includes an alloy represented by the general formula (III):M_(2−x)M2_(x)M1_(y) wherein M is at least one element selected Be, Ca,Sr, Ba, Y, Ra, La, Ce, Pr, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu, Ti,Zr, Hf, Pd and Pt; M2 is at least one element selected (excluding M)from the group consisting of elements which are capable of causing anexothermic reaction with hydrogen, Al and B; M2 is at least one elementselected (excluding Mg and M2) from elements which are incapable ofcausing an exothermic reaction with hydrogen; x is defined as0.01<x≦1.0; and y is defined as 0.5<y≦1.5. Namely, since thehydrogen-absorbing alloy represented by the general formula (III) ischaracterized in that part of M such as Zr is substituted with anelement represented by M2 such as Al, the hydrogen-absorbing property,in particular the lowering of hydrogen-absorbing temperature can beimproved as compared with the conventional A₂B type hydrogen-absorbingalloy, and at the same time a high hydrogen-absorbing capacity inherentto the A₂B type hydrogen-absorbing alloy can be maintained. Further,this hydrogen-absorbing alloy represented by the general formula (III)is larger in hydrogen-absorbing capacity (based on weight), lower inmanufacturing cost and lighter in weight as compared with theconventional rare earth-based hydrogen-absorbing alloy.

Therefore, the hydrogen-absorbing alloy represented by the generalformula (III) according to this invention is capable of loweringhydrogen-absorbing temperature and at the same time capable ofsustaining a high hydrogen-absorbing capacity inherent to the A₂B typehydrogen-absorbing alloy. Moreover, since the hydrogen-absorbing alloyrepresented by the general formula (III) is capable of sustaining a highhydrogen-absorbing capacity inherent to the A₂B type hydrogen-absorbingalloy, the alloy would be quite useful as an electrode material of asecondary battery.

Another example of the hydrogen-absorbing alloy according to the presentinvention contains an alloy represented by the following general formula(V):

(Mg_(1−x)M3_(x))_(20−y)M4  (V)

wherein M4 is at least one element selected Ni, Fe, Co, Cu, Zn, Sn andSi; M3 is at least one element selected (excluding the elements of M4)from the group consisting of elements which are more electronegativethan Mg; x is defined as 0<x<0.5; and y is defined as 0≦y<18.

Preferable examples of M3, i.e. elements (excluding the elements of M4)which are more electronegative than Mg are Al (1.5), Mn (1.5), Ta (1.5),V (1.6), Cr (1.6), Nb (1.6), Ga (1.6), In (1.7), Ge (1.8), Pb (1.8), Mo(1.8), Re (1.9), Ag (1.9), B (2.0), C (2.5), P (2.1), Ir (2.2), Rh(2.2), Ru (2.2), Os (2.2), Pt (2.2), Au (2.4), Se (2.4), S (2.5), Sc(1.3), Zr (1.4), Hf (1.3), Pd (1.8) and Tl (1.8). The number in theparenthesis indicates an electronegativity of a metal which is obtainedfrom a value of poling of each metal element. These elements may beemployed singly or in combination thereof.

The reasons for limiting the ranges of x and y in the general formula(V) are as follows. Namely, when the value of x exceeds 0.5, the crystalstructure of alloy would be extremely altered and at the same time theinherent properties of the Mg-based alloy would be deteriorated.Preferable range of x is 0.01≦x≦0.4. The alloy having the value xconfined in this preferable range would exhibit an increased amount ofhydrogen-absorption. On the other hand, when the value of y exceeds 18,the site for hydrogen absorption in the alloy would be decreased so thatthe amount of hydrogen-absorption would be decreased. Preferable rangeof y is 1≦y≦17.5.

This hydrogen-absorbing alloy comprising an alloy represented by thegeneral formula (V) and explained above exhibits excellenthydrogen-absorbing and hydrogen-releasing properties.

Followings are observations made from the viewpoint of electronegativityon any change in bonding strength between an alloy and hydrogen, i.e.any change in stability of hydrogen in the alloy when the Mg componentin the aforementioned general formula (V) is substituted by the M3element such as Pt and Zr.

Generally, there is a relationship in most of the hydride of elementalmetal that as the difference in electronegativity between an alloy andhydrogen becomes larger, the bonding strength of the metal-hydrogen bondbecomes higher. It is also considered that as the difference inelectronegativity between an alloy and hydrogen becomes larger, theionic bonding property of the metal-hydrogen bond is increasinglyenhanced, thereby strengthening the metal-hydrogen bond and resulting inan increased stability of the absorbed hydrogen. In other words, when Mgis substituted by the M3 element such as Al and Ag, i.e. by a metalhaving a larger electronegativity than Mg, the difference inelectronegativity between such a metal and hydrogen becomes smaller, sothat hydrogen within the crystal lattice is assumed to be instabilized.

Therefore, it is possible, when the site of Mg is substituted by the M3element which is more electronegative than Mg, such as Al and Ag toinstabilize hydrogen atoms in the crystal lattice, thereby to improvethe hydrogen absorption properties of the alloy and to facilitate themanufacture of the alloy.

On the other hand, M4 in the aforementioned general formula (V) such asNi is effective in improving the hydrogen absorption properties as wellas in enhancing the release of hydrogen which has been absorbed in thealloy, since M4 is more electronegative than Mg and inherently incapableof exothermally reacting with hydrogen, i.e. inherently incapable ofspontaneously forming a hydride.

As explained above, the hydrogen-absorbing alloy according to thisinvention which comprises an alloy represented by the general formula(V) is featured in bringing about a prominent improvement on thehydrogen absorption properties, in particular the amount of hydrogenabsorption as compared with the conventional Mg₂Ni-type alloy. Thehydrogen-absorbing alloy according to this invention is also featured inhaving practical advantages that, as compared with the conventional rareearth element type hydrogen-absorbing alloy, the hydrogen-absorbingalloy of this invention is larger in amount of hydrogen absorption perweight, cheaper in manufacturing cost and lighter in weight.

Another example of the hydrogen-absorbing alloy according to the presentinvention contains an alloy represented by the following general formula(VI):

(Mg_(1−x)M5_(x))_(20−y)M6  (VI)

wherein M5 is at least one element (excluding elements which are moreelectronegative than Mg) which has an atomic radius 1 to 1.5 times ashigh as that of Mg; M6 is at least one element selected Ni, Fe, Co, Cu,Zn, Sn and Si; x is defined as 0<x<0.5; and y is defined as 0≦y<18.

If the atomic radius of M5 element is larger than Mg by more than 1.5times, it may become difficult to form a single alloy phase, resultingin a deterioration of hydrogen absorption properties of the alloy. Asfor the elements representing the aforementioned M5, Ca, Sr, K and Namay be employed. Among these M5 elements, more preferable examplesthereof are Ca and Sr.

The reasons for limiting the ranges of x and y in the general formula(VI) are as follows. Namely, when the value of x exceeds 0.5, thecrystal structure of alloy would be extremely altered and at the sametime the inherent properties of the Mg-based alloy would bedeteriorated. Preferable range of x is 0.01≦x≦0.4. The alloy having thevalue x confined in this preferable range would exhibit an increasedamount of hydrogen-absorption. On the other hand, when the value of yexceeds 18, the site for hydrogen absorption in the alloy would bedecreased so that the amount of hydrogen-absorption would be decreased.Preferable range of y is 1≦y≦17.5.

This hydrogen-absorbing alloy comprising an alloy represented by thegeneral formula (VI) and explained above exhibits excellenthydrogen-absorbing and hydrogen-releasing properties.

Namely, when the Mg site in the alloy is substituted by the M5 elementshaving 1 to 1.5 times as high atomic radius as that of Mg, such as Sr,the catalytic activity to hydrogen of the alloy can be increased,thereby improving the hydrogen absorption properties.

On the other hand, M6 in the aforementioned general formula (VI) such asNi is effective in improving the hydrogen absorption properties as wellas in enhancing the release of hydrogen which has been absorbed in thealloy, since M6 is more electronegative than Mg and inherently incapableof exothermally reacting with hydrogen, i.e. inherently incapable ofspontaneously forming a hydride.

As explained above, the hydrogen-absorbing alloy according to thisinvention which comprises an alloy represented by the general formula(VI) is featured in bringing about a prominent improvement on thehydrogen absorption properties, in particular the amount of hydrogenabsorption as compared with the conventional Mg₂Ni-type alloy. Thehydrogen-absorbing alloy according to this invention is also featured inhaving practical advantages that, as compared with the conventional rareearth element type hydrogen-absorbing alloy, the hydrogen-absorbingalloy of this invention is larger in amount of hydrogen absorption perweight, cheaper in manufacturing cost and lighter in weight.

The method of modifying the surface of a hydrogen-absorbing alloyaccording to the present invention comprises a step of treating thehydrogen-absorbing alloy with an R—X compound, wherein R representsalkyl, alkenyl, alkynyl, aryl or a substituted group thereof; X is ahalogen element.

Examples of the hydrogen-absorbing alloy are (1) an AB₅ type (forexample, LaNi₅, CaNi₅); (2) an AB₂ type (for example, MgZn₂, ZrNi₂); (3)an AB type (for example, TiNi, TiFe); and (4) an A₂B type (for example,Mg₂Ni, Ca₂Fe).

The method according to this invention is used anotherhydrogen-absorbing alloy containing an alloy represented by thefollowing formula (IV):

Mg_(2−x)M2_(x)M1_(y)  (IV)

wherein M2 is at least one element selected (excluding M) from the groupconsisting of elements which are capable of causing an exothermicreaction with hydrogen, Al and B; M1 is at least one element selected(excluding Mg and M2) from elements which are incapable of causing anexothermic reaction with hydrogen; x is defined as 0≦x≦1.0; and y isdefined as 0.5<y≦2.5.

As examples of M1 in the formula (IV), the same elements as explainedwith reference to the alloy represented by the general formula (I) maybe employed.

As examples of M2 in the formula (IV), the same elements as explainedwith reference to the alloy represented by the general formula (II) maybe employed.

In the method according to this invention, a hydrogen-absorbing alloy ofthe AB₅ type or the A₂B type as mentioned above, or a hydrogen-absorbingalloy containing an alloy represented by the general formula (IV) ispreferably used.

In the above mentioned R—X compound, R represents alkyl, alkenyl,alkynyl, aryl or a substituted group thereof, and X is a halogenelement, the reactivity of which being in the order of aniodide>bromide>chloride. Examples of such an R—X compound are methyliodide, ethyl bromide, 1,2-dibromoethane and 1,2-diiodoethane.

This R—X compound (a halide) is preferably employed to react with ahydrogen-absorbing alloy under the presence of a solvent therebymodifying the surface of the hydrogen-absorbing alloy.

Examples of this solvent are diethyl ether, tetrahydrofuran (THF),di-n-propyl ether, di-n-butyl ether, di-n-isopropyl ether,diethylglycoldiethyl ether (diglyme), dioxane and dimethoxyethane (DME).These solvents may be employed singly or in combination. Preferablesolvents among them are diethyl ether and THF. In the case where the R—Xcompound is alkyl halide, alkenyl halide or aryl halide, an ether typesolvent may be preferably employed as a solvent. On the other hand, theR—X compound selected is an alkenyl compound or an aryl compound, THFwhich has higher coordinating strength can be preferably employed. Whenthe reaction is to be performed in an ether, bromides and iodides amongthese R—X compounds can be more easily put into the reaction. Chloridesor substituted bromides, which are relatively low in reactivity amongthese R—X compounds can be put into the reaction using THF.

The concentration of a solution dissolving the above mentioned R—Xcompound with the above mentioned solvent should be suitable determinedtaking the following points into consideration.

(1) Reactivity of a halide (when a halide of low reactivity is used, thehalide should be incorporated in a higher concentration).

(2) Possibility of causing a side reaction (when aryl chloride or benzylchloride is use, a coupling reaction may possibly be caused, so thatthese halides should be incorporated in a lower concentration).

(3) Solubility and stability of products (when the solubility of ahalide is low, the halide should be incorporated in a lowerconcentration. Namely, when the solution has a saturated concentrationor more, a solid substance may be precipitated upon cooling therebygiving rise to heterogeneity).

In order to facilitate the reaction, a catalyst may preferably added tothe solution having the R—X compound dissolved therein. Examples of sucha catalyst are condensated polycyclic hydrocarbons such as pentalene,indene, naphthalene, azulene, heptalene, biphenylene, indacene,acenaphthylene, fluorene, phenalene, phenanthrene, anthracene,fluoranthene, acephenanthrylene, anthrylene, triphenylene, pyrene,chrysene, naphthacene, pleiadene, picene, perylene, pentaphene,tentacene, tetraphenylene, hexaphene, hexacene, rubicene, coronene,trinaphthylene, pentaphene, heptacene, pinanthrene and ovalene. Amongthese compounds, anthracene is most preferred. When a hydrogen-absorbingalloy comprising Mg is treated with a THF solution of R—X compound withanthracene added therein, a mixture of anthracene and magnesium acts toform an equilibrium between magnesium-anthracene. Therefore, a mereaddition of anthracene as a catalyst into the reaction systemrepresented by a reaction formula (3) described hereinafter wouldpromote a reaction of the rightward direction thereby making it possibleto perform an excellent surface modification of the alloy.

It would be advantageous in the industrial application of the method ofsurface-modification according to this invention to add R—MS (M: acomponent in a hydrogen-absorbing alloy) prepared in advance to thereaction system in the initial stage and to perform a dehydration andactivation of the reaction system.

According to the method of surface-modification of hydrogen-absorbingalloy as proposed by this invention, it is possible to improve thehydrogen-absorbing property and activity in particular of ahydrogen-absorbing alloy as compared with those which are notsurface-modified.

Therefore, one of the methods of improving the hydrogen-absorbingproperty of a hydrogen-absorbing alloy is to modify the surface of thehydrogen-absorbing alloy. Namely, the activity of a hydrogen-absorbingalloy in a process of hydrogen absorption is occurred by a mechanism ofsurface segregation, so that this activity is considered to be relatedto the easiness of forming a catalytic layer on the surface of the alloyand the performance of catalyst. When a hydrogen-absorbing alloy istreated with an R—X compound, M constituting part of component elementsof the hydrogen-absorbing alloy undergoes the following reactions.

M+R—X→R—MX  (1)

or

M+αR—x→αR—MX_(α)  (2)

When the surface treatment is performed through these reactions, agentle segregation is caused to occur on or near the surface of thehydrogen-absorbing alloy thereby producing an active site acting as acatalyst thereon.

For instance, when a Mg₂Ni type hydrogen-absorbing alloy is treated withethyl bromide, magnesium in the hydrogen-absorbing alloy undergoes thefollowing reaction.

Mg+C₂H₅Br→C₂H₅—MgBr  (3)

With this reaction (surface modification), the oxide film covering thesurface of Mg₂Ni is removed thereby exposing nickel to be functioned asa hydrogen-dissociating catalyst at the time of hydrogen-absorption thusimproving the hydrogen-absorbing property of the alloy.

Examples of the element (other than magnesium) which is capable ofreacting with the R—X compound are rare earth elements Ln (Ln:lanthanum, cerium, praseodymium, neodymium, promethium, samarium,europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium,ytterbium and ruthenium). When these Ln elements are allowed to reactwith 1,2-diiodoethane in THF in an atmosphere of argon or nitrogen gasat room temperature, LnI₂ is produced according to the followingreaction formula.

Ln+ICH₂CH₂I→LnI₂+CH₂═CH₂  (4)

La, Nd, Sm and Lu in particular among these rare earth elements are morereactive to 1,2-diiodoethane, the reactivity thereof being in the orderof La>Nd>Sm>Lu.

For instance, when a LaNi₅ type hydrogen-absorbing alloy is treated with1,2-diiodoethane, lanthanum in the hydrogen-absorbing alloy undergoesthe following reaction.

La+ICH₂CH₂I→LaI₂+CH₂═CH₂  (5)

With this reaction (surface modification), the oxide film covering thesurface of LaNi₅ is removed thereby exposing nickel to be functioned asa hydrogen-dissociating catalyst at the time of hydrogen-absorption thusimproving the hydrogen-absorbing property of the alloy.

The amount of halogen left remained on the surface of hydrogen-absorbingalloy after the treatment with a solution of the R—X compound is only alittle (for example, not more than 1%), so that the property inherent tothe hydrogen-absorbing alloy would not be substantially diminished.

A hydrogen-absorbing alloy according to the another present invention ischaracterized in that a half-width Δ(2θ) of at least one peak out ofpeaks of three strongest lines to be obtained by an X-ray diffractionusing Cukα-ray as a radiation source lies in the range of0.2°≦Δ(2θ)≦50°. The term of three strongest lines used herein isintended to mean three largest peak as counted from the largest peak inan X-ray diffraction pattern.

Examples of the hydrogen-absorbing alloy composition are (1) an AB₅ type(for example, LaNi₅, CaNi₅); (2) an AB₂ type (for example, MgZn₂,ZrNi₂); (3) an AB type (for example, TiNi, TiFe); and (4) an A₂B type(for example, Mg₂Ni, Ca₂Fe).

The hydrogen-absorbing alloy is preferably a composition containing analloy represented by the following formula (IV):

Mg_(2−x)M2_(x)M1_(y)  (IV)

wherein M2 is at least one element selected (excluding M) from the groupconsisting of elements which are capable of causing an exothermicreaction with hydrogen, Al and B; M1 is at least one element selected(excluding Mg and M2) from elements which are incapable of causing anexothermic reaction with hydrogen; x is defined as 0≦x≦1.0; and y isdefined as 0.5<y≦2.5.

As examples of M1 in the formula (IV), the same elements as explainedwith reference to the alloy represented by the general formula (I) maybe employed.

As examples of M2 in the formula (IV), the same elements as explainedwith reference to the alloy represented by the general formula (II) maybe employed.

In particular, in the case of a hydrogen-absorbing alloy containing 10%or more of magnesium such as a Mg₂Ni type hydrogen-absorbing alloy or ahydrogen-absorbing alloy containing an alloy represented by the generalformula (IV), the alloy should preferably be characterized in that anapparent half-width Δ(2θ₁) of a peak in the vicinity of 20° lies in therange of 0.3°≦Δ(2θ₁)≦10°, or an apparent half-width Δ(2θ₂) of a peak inthe vicinity of 40° lies in the range of 0.3° ≦Δ(2θ₂)≦10° in an X-raydiffraction using Cukα-ray as a radiation source.

The reasons for limiting the range of Δ(2θ) are as follows. Namely, ifthis Δ(2θ) is less than 0.2°, the rate of absorbing hydrogen wouldbecome too slow. On the other hand, if this Δ(2θ) exceeds over 50°, thehydrogen-absorbing capacity of the alloy would be reduced. Morepreferable half-width Δ(2θ) is in the range of 0.3°≦Δ(2θ)≦10°.

When the size of crystal grain of the hydrogen-absorbing alloy isrepresented by D, it is preferable to confine the range of D to 0.8nm≦D≦50 nm. When the hydrogen-absorbing alloy having D confined in thisrange is used, the path of hydrogen diffusion may be enlarged and thedistance of path may be minimized so that the hydrogen-absorbing anddesorbing property of the alloy can be improved. The reasons forlimiting the range of crystal grain are as follows. Namely, if this D isless than 0.8 nm, the hydrogen-absorbing capacity of the alloy may belowered. On the other hand, if this D is larger than 50 nm, the path ofhydrogen diffusion would be inhibited.

A method of modifying the surface of a hydrogen-absorbing alloyaccording to another embodiment of this invention comprises a step ofmechanically treating a hydrogen-absorbing alloy under vacuum or in anatmosphere of an inert gas or hydrogen.

Regarding the mechanical treatment to be adopted in the surfacemodification mentioned above, the following method may be taken. Namely,at first, a hydrogen-absorbing alloy is introduced into a vessel havingballs disposed therein such as an epicyclic ball mill, screw ball mill,rotational ball mill, or attriter, and then a mechanical impact is givento the hydrogen-absorbing alloy through collisions between the innerwall of the vessel and the balls, or between balls.

When this mechanical treatment is to be carried out with the vesselbeing sealed, the treatment may be performed in an apparatus such as adry box filled therein with an argon or inert gas atmosphere, or in avessel provided with a discharge valve for exhausting the interior ofthe vessel. It is also possible in some case to carry out the treatmentwith the vessel filled with hydrogen gas. When this sealed vessel isemployed, since the impact is effected throughout the interior of thevessel, the sealed portion thereof is also subjected to the impactthereby causing a loosening the sealed portion. Therefore, it may beadvisable, in order to secure the hermetic condition of the vessel, tomake the cover of the vessel into a double cover, or to dispose thevessel in an inert atmosphere or in a vacuum chamber. It is alsopreferable to control the purity of the inert gas in the preparation ofan inert gas atmosphere. For example, it is desirable to control theinert gas atmosphere to contain not more than 100 ppm of oxygen and notmore than 50 ppm of water vapor. The hydrogen-absorbing alloy particlesas well as metal particles. should be treated in an inert gas atmosphereso as to avoid the oxidization thereof.

The mechanical treatment using an epicyclic ball mill and the likeshould be carried out for 1 to 1,000 hours. If the treating time is lessthan one hour, it may be difficult to obtain a desiredhydrogen-absorbing property of hydrogen-absorbing alloy. On the otherhand, if the treating time exceeds over 1,000 hours, a gradual oxidationwould proceed and the manufacturing cost would be raised.

The particle size of the hydrogen-absorbing alloy modified by thismechanical treatment should desirably be in the range of 0.1 to 50 μm.If necessary, a heat treatment may also be performed on this modifiedhydrogen-absorbing alloy. The heat treatment temperature is determined acomposition of a hydrogen-absorbing alloy. This temperature in this caseshould preferably be in the range of about 100 to 500° C. There is apossibility in some case that the, element components incorporated inthe mixture may be coagulated with the alloy as a result of themechanical treatment. This agglomeration may be effective for improvingthe hydrogen-absorbing property of the alloy. In this case, a desirableratio of this agglomeration is 10% by weight or more.

According to the method of modifying the surface of a hydrogen-absorbingalloy of this invention, it is possible to remarkably improve theinitial activity and hydrogen-absorbing property of a hydrogen-absorbingalloy. This method comprises a step of mechanically treating ahydrogen-absorbing alloy under vacuum or in an atmosphere of an inertgas or hydrogen.

The improvement of the hydrogen-absorbing property of ahydrogen-absorbing alloy may be effected by any of following methods:(1) a modification through coating; (2) a modification throughtopochemical method; (3) modification through mechanochemical method;(4) a modification through an encapsulation; and (5) co-use of radiationexposure. According to the surface modification of this invention, thethird method, i.e., (3) modification through mechanochemical method, isselected out of these various methods, thereby succeeding to remarkablyimprove the initial activation and hydrogen-absorbing property of ahydrogen-absorbing alloy.

According to this mechanical modification method of this invention, itis possible not only to alter the fundamental structure of alloyparticles, but also to alter the physical property of alloy particlesthrough the change in surface structure. In other words, it is possibleto alter the internal energy of hydrogen-absorbing alloy. Moreover, itis possible to increase the surface energy through the generation offresh surface resulting from the generation of fine grains.

According to this mechanical modification method of this invention, thesurface and structure of a hydrogen-absorbing alloy are altered therebygenerating a stress and thus causing a structural destruction, adisplacement of atom or molecule, and a deterioration of crystalregularity, resulting in an increases of potential energy. By making themost of these various effects, it is possible to improve a catalyticactivity and a selectivity in the catalytic reaction.

The lattice defect within a hydrogen-absorbing alloy may be pointed outas one of phenomena resulting from the modification. In this case, thelattice defect may include, in addition to a lattice defect which isthermodynamically allowable in an ideal crystal, a plastic deformationformed in a crystal grain by a mechanical energy generated in thesurface modification process, a difference in temperature in the crystalgrain or phase transition due to a local generation of heat, and aresidual stress resulted from a generation of phase transition. It isalso possible to improve the hydrogen-absorbing property of ahydrogen-absorbing alloy by making the most of the effects obtainablefrom these phenomena.

An expansion of the profile of peaks has been confirmed through themeasurement of peaks in X-ray diffraction pattern of a sample(hydrogen-absorbing alloy) using Cukα as a radiation source after amechanical modification has been effected to the samples. Generally, theexpansion of the profile of peaks can be ascribed to (a) a change insize of crystal grain; and (b) a heterogeneous stain.

Regarding (a), the size D of crystal grain can be represented by theScherrer's equation as follows.

D=(0.9λ)/(Δ(2θ)cos θ)  (6)

D: Size of crystal grain;

Δ(2θ): Apparent half-width;

λ: Wavelength of X-ray employed;

θ: Bragg angle of diffraction line;

In the same manner, the size ε of crystal grain can be represented bythe Stokes and Wilson's equation as follows.

ε=λ/(βi cos θ)  (7)

ε: Size of crystal grain;

β_(i): Integrated width;

λ: Wavelength of X-ray employed;

θ: Bragg angle of diffraction line;

When a hydrogen-absorbing alloy is mechanically treated as explainedabove, the peaks in X-ray diffraction pattern are caused to becomebroader. This can be attributed, in view of these two equationsmentioned above, to the fact that the size of crystal grain becomessmaller as a result of the mechanical treatment.

The influence by (b) the stain of crystal grains should also be takeninto consideration as one of the reasons of causing the broadening ofX-ray diffraction peaks. This stain of crystal grain may be ascribed toa change or fluctuation of an interval between crystal planes. From theStokes and Wilson's equation, the relationship between the heterogeneousstain η of crystal grain and the integrated width β′_(i) of diffractionline based on the heterogeneous stain η can be expressed as follows.

β′_(i)=2η tan θ  (8)

Further, the expansion of the profile due to both of the size of crystalgrain and the heterogeous stain can be represented according to Hall'sequation by the following equation.

β=β_(i)+β′_(i)  (9)

Therefore, the expansion of profile in the hydrogen-absorbing alloyafter the mechanical treatment thereof can be ascribed to both of thechange in crystal grain size and the generation of the heterogeneousstain. Accordingly, the hydrogen-absorbinq property of the alloy can beimproved by controlling these two factors. Namely, when the crystalgrain of a hydrogen-absorbing alloy is made smaller in size through themechanical treatment, the diffusion path of hydrogen can be enlarged,with its distance being made shortened. As a result, thehydrogen-absorbing and desorbing property of a hydrogen-absorbing alloycan be improved. The preferable size D of such a crystal grain is in therange of 0.8 nm≦D≦50 nm. Further, the impact energy resulting from themechanical treatment is great enough to alter the interval between theplanes of hydrogen-absorbing alloy particles thereby causing a latticeasymmetry in the hydrogen-absorbing alloy. In other words, since it ispossible to generate a crystal stain in the hydrogen-absorbing alloy,the hydrogen-absorption and desorption can be easily effected bychanging the energy within the crystal lattice.

In a hydrogen-absorbing alloy undergone through the mechanicaltreatment, the profile expansion represented by the apparent half-widthΔ(2θ) as shown in the equation (6) would become in the range of0.2°≦Δ(2θ)≦50, more preferably 0.3°≦Δ(2θ)≦10°.

Another example of the hydrogen-absorbing alloy according to the presentinvention is formed of a mixture comprising:

an alloy having hydrogen-absorbing properties; and

at least one additive selected from the group consisting of (a) at leastone element selected from Group IA elements (such as Li, Na, Rb, Ca, Sr,Ba, etc.), Group IIA elements (such as Be, Mg, Rb, Cs, etc.), Group IIIAelements (such as Sc, Y, etc.), Group IVA elements (Ti, Zr and Hf), VAelements (V, Nb and Ta), Group VIA elements (Cr, Mo and W), Group VIIAelements (such for example as Mn, Re, etc.), Group VIIIA elements (Fe,Ru, Os, Co, Rh, Ir, Ni, Pd and Pt), Group IB elements (Cu, Ag and Au),Group IIB elements (Zn, Cd and Hg), Group IIIB elements (such as B, Al,Ga, In, Tl, etc.), Group IVB elements (C, Si, Ge, Sn and Pb), Group VBelements (P, As, Sb and Bi), and Group VIB elements (such as S, Se, Te,etc.); (b) an alloy formed of any combination of elements defined insaid (a); and (c) an oxide of any of elements defined in said (a); theaforementioned mixture being mechanically treated under vacuum or in anatmosphere of an inert gas or hydrogen.

As for the aforementioned alloy having hydrogen-absorbing properties, anA₂B alloy (wherein A is an element which is capable of causing anexothermic reaction with hydrogen, and B is an element which isincapable of causing an exothermic reaction with hydrogen) or alloysrepresented by the aforementioned general formulas (IV) to (VI) may beemployed. In particular, hydrogen-absorbing alloys containing an alloyrepresented by the aforementioned general formula (V) or (VI) arepreferable.

The elements of aforementioned (a), alloys of aforementioned (b) andoxides of aforementioned (c) all function as a catalytic seed forhydrogenation as they are forcibly attached to the alloy havinghydrogen-absorbing properties.

Preferable elements among the aforementioned (a) are those which exhibita high catalytic activity to the reaction thereof with hydrogen. Namely,an element which is positive (endothermic) in heat of reaction withhydrogen, or an element which exhibits a larger exchange current densityi₀ in the reaction at the negative electrode (hydrogen electrode), ifthe application of the element to a buttery is taken into account, ispreferable. Examples of such an element are V, Nb, Ta, Cr, Mo, W, Mn,Fe, Ru, Co, Rh, Ir, Pd, Ni, Pt, Cu, Ag, Au, etc.

As for the alloy defined in the aforementioned (b), those which exhibita higher catalytic activity due to the synergistic effect between theconstituent elements of the alloy in the reaction at the hydrogenelectrode than the catalytic activity to be derived from eachconstituent element are preferable. Specific examples of thesepreferable alloys are. Ni—Ti type alloy, Ni—Zr type alloy, Co—Mo typealloy, Ru—V type alloy, Pt—W type alloy, Pd—W type alloy, Pt—Pd typealloy, V—Co type alloy, V—Ni type alloy, V—Fe type alloy, Mo—Co typealloy, Mo—Ni type alloy, W—Ni type alloy and W—Co type alloy. Amongthese alloy, MoCo₃, WCo₃, MoNi₃ and WNi₃ are more preferable, sincethese alloys are high in catalytic activity and suited for improving thehydrogen-absorbing property of the hydrogen-absorbing alloy.

With respect to oxides defined in the aforementioned (c), an oxide whichis capable of giving a large exchange current density i₀ is preferable.Preferable examples of such an oxide are FeO, RuO₂, CoO, Co₂O₃, Co₃O₄,RhO₂, IrO₂ and NiO.

The aforementioned mixture should preferably be formed of a compositionwhere the aforementioned additive is contained in the ratio of 0.01 to70% by volume based on the volume of the alloy having hydrogen-absorbingproperties. If the content of this additive is less than 0.01% byvolume, it may be difficult to improve the rate of hydrogen absorptionby the hydrogen-absorbing alloy. On the other hand, if the content ofthis additive exceeds 70% by volume, the amount of hydrogen absorptionby the hydrogen-absorbing alloy may be decreased. More preferablycontent of the additive is in the range of 1 to 50% by volume.

Regarding the mechanical treatment to be adopted in the surfacemodification mentioned above, the same apparatus as explained above,i.e. such as an epicyclic ball mill, screw ball mill, rotational ballmill, or attritor may be employed.

This mechanical treatment by making use of an epicyclic ball mill forinstance may be performed in the same manner as explained above within atime period of 1 to 1,000 hours.

Since the hydrogen-absorbing alloy according to this invention is formedof a mixture comprising an alloy having hydrogen-absorbing properties;and at least one additive selected from the group consisting of (a) atleast one element selected from Group IA elements, Group IIA elements,Group IIIA elements, Group IVA elements, VA elements, Group VIAelements, Group VIIA elements, Group VIIIA elements, Group IB elements,Group IIB elements, Group IIIB elements, Group IVB elements, Group VBelements and Group VIB elements; (b) an alloy formed of any combinationof elements defined in said (a); and (c) an oxide of any of elementsdefined in said (a); the resultant mixture being mechanically treatedunder vacuum or in an atmosphere of an inert gas or hydrogen, theinitial activity and the hydrogen-absorbing properties of thehydrogen-absorbing alloy can be prominently improved.

Namely, the activity of the hydrogen absorptive alloy per se at themoment of absorbing hydrogen is considered to be influenced by theeasiness of forming a catalytic layer and also by the performance of thecatalytic layer. In view of these facts, it is possible to improve thehydrogen-absorbing properties of the hydrogen-absorbing alloy by takingadvantage of the effects as explained in the aforementioned method ofmodifying the surface of the hydrogen-absorbing alloy.

Accordingly, in the case of the hydrogen-absorbing alloy according tothis invention, with a view to further improve the hydrogen-absorbingproperties of the hydrogen absorptive alloy, the aforementionedadditives are added at the ratio of 0.01 to 70% by volume to the alloyhaving hydrogen-absorbing properties, and the resultant mixture issubjected to a mechanical treatment under vacuum, or in an atmosphere ofan inert gas or hydrogen, thereby attaching a catalytic seed for thehydrogenation. It is possible, with this mechanical treatment, toforcibly attach a catalytic seed to be functioned as a hydrogenationcatalyst at the moment of hydrogen absorption to the surface or vicinityof surface of the alloy having hydrogen-absorbing properties, and henceto improve the initial activity of the alloy. Therefore, it is possibleaccording to this hydrogen-absorbing alloy to prominently improve theinitial activity and the hydrogen-absorbing properties of thehydrogen-absorbing alloy.

For example, when Ni is mixed at a predetermined ratio with a Mg₂Ni typealloy having hydrogen-absorbing properties, and the resultant mixture issubjected to a mechanical treatment in an atmosphere of an inert gassuch as argon, nickel to be functioned as a dissociation catalyst forhydrogen at the moment of hydrogen absorption would be attached onto thesurface of the alloy. As a result, it is possible to improve thehydrogen-absorbing properties of the hydrogen-absorbing alloy. Moreover,when this mechanical treatment is performed in this manner, the particlediameter of the alloy would be decreased, thereby increasing the ratioof grain boundary of the alloy and giving rise to the generation ofnon-uniform distortion within the crystal, thereby making it possible tofacilitate the absorption of hydrogen.

Another example of the hydrogen-absorbing alloy according to thisinvention comprises an alloy having hydrogen-absorbing properties; and0.01 to 70% by volume of at least one powdered additive having 0.01 to100 μm in average diameter, which is dispersed in said alloy andselected from the group consisting of (a) at least one element selectedfrom Group IA elements (such as Li, Na, Rb, Ca, Sr, Ba, etc.), Group IIAelements (such as Be, Mg, Rb, Cs, etc.), Group IIIA elements (such asSc, Y, etc.), Group IVA elements (Ti, Zr and Hf), VA elements (V, Nb andTa), Group VIA elements (Cr, Mo and W), Group VIIA elements (such forexample as Mn, Re, etc.), Group VIIIA elements (Fe, Ru, Os, Co, Rh, Ir,Ni, Pd and Pt), Group IB elements (Cu, Ag and Au), Group IIB elements(Zn, Cd and Hg), Group IIIB elements (such as B, Al, Ga, In, Tl, etc.),Group IVB elements (C, Si, Ge, Sn and Pb), Group VB elements (P, As, Sband Bi), and Group VIB elements (such as S, Se, Te, etc.); (b) an alloyformed of any combination of elements defined in said (a); and (c) anoxide of any of elements defined in said (a).

As for the aforementioned alloy having hydrogen-absorbing properties, anA₂B alloy (wherein A is an element which is capable of causing anexothermic with hydrogen, and B is an element which is incapable ofcausing an exothermic reaction with hydrogen) or alloys represented bythe aforementioned general formulas (IV) to (VI) may be employed. Inparticular, hydrogen-absorbing alloys containing an alloy represented bythe aforementioned general formula (V) or (VI) are preferable.

All of the additives, i.e. the elements of aforementioned (a), alloys ofaforementioned (b) and oxides of aforementioned (c) function as acatalytic seed for hydrogenation as they are forcibly attached to thealloy having hydrogen-absorbing properties. As for these additives, thesame kinds of materials as explained above with reference to thehydrogen-absorbing alloy of this invention would employed.

The reasons for limiting the size of the powdered additives are asfollows. Namely, if the size of the powdered additives is less than 0.01μm, it may become difficult to improve the hydrogen-absorbing propertiesof the hydrogen-absorbing alloy. On the other hand, if the size of thepowdered additives exceeds 100 μm, the rate of hydrogen absorption bythe hydrogen-absorbing alloy may be lowered. More preferable particlesize of the powdered additives is in the range of 0.1 to 50 μm.

The reasons for limiting the dispersion volume of the powdered additivesto the alloy having hydrogen-absorbing properties are as follows.Namely, if the dispersion volume of the powdered additives is less than0.01% by volume, it may be difficult to promote the rate of hydrogenabsorption by the hydrogen-absorbing alloy. On the other hand, if thedispersion volume of this additive exceeds 70% by volume, the amount ofhydrogen absorption by the hydrogen-absorbing alloy may be decreased.More preferably dispersion volume of the additive is in the range of 1to 50% by volume.

As for the method of dispersing the aforementioned powdered additivesinto the alloy having hydrogen-absorbing properties, various methodssuch as a method of performing the aforementioned mechanical treatmentafter the mixing of the additive with the alloy; a method of adding theadditive at the occasion of melting the alloy; an ultra-quenchingmethod; an atomizing method; a plating method; a CVD method;

a sputtering method; a mechanical alloying method; a rolling method; asol-gel method may be employed.

According to this another example of the hydrogen-absorbing alloy ofthis invention, the aforementioned additive having a predeterminedparticle diameter is dispersed into the alloy having hydrogen-absorbingproperties at a ratio of 0.01 to 70% by volume, so that a catalytic seedfor the hydrogenation is attached to the alloy, thus prominentlyimproving the initial activity of the alloy. Therefore, it is possibleaccording to this hydrogen-absorbing alloy to prominently improve theinitial activity and the hydrogen-absorbing properties of thehydrogen-absorbing alloy.

An alkali secondary battery according to the present invention will beexplained further with reference to a cylindrical nickel-hydrogensecondary battery.

Referring to FIG. 3, a bottomed cylindrical case 1 is accommodatedtherein with an electrode group 5 which has been manufactured byspirally winding a stacked body comprising a positive electrode 2, aseparator 3 and a negative electrode 4. The negative electrode 4 isdisposed at the outermost periphery of the electrode group 5 so as toelectrically contact with the cylindrical case 1. The cylindrical case 1is also filled with an alkali electrolyte. A sealing plate 7 formed of adisk having an opening 6 at the center is disposed on the upper openingof the cylindrical case 1. An insulating gasket 8 having a ring-likeshape is interposed between the periphery of the sealing plate 7 and theupper inner wall surface of the opening of the cylindrical case 1. Theperipheral fringe portion of the opening of the cylindrical case 1 iscaulked inward so that the sealing plate 7 is hermetically fixed via thegasket 8 to cylindrical case 1. The positive electrode lead 9 isconnected through its one end to the positive electrode 2 and throughits other end to the lower surface of the sealing plate 7. A positiveterminal 10 having a hat-like shape is mounted over the sealing plate 7in such a manner as to cover the opening 6. A rubber safety valve 11 isdisposed in a space surrounded by the sealing plate 7 and the positiveelectrode terminal 10 in such a manner as to seal the opening 6. Aholding plate 12 formed of an insulating disk having an opening isdisposed over the positive electrode terminal 10 in such a manner thatthe projected portion of the positive electrode terminal 10 is protrudedout through the opening of the holding plate 12. An envelope tube 13 isdisposed to cover all of the periphery of the holding plate 12, the sidewall of the cylindrical case 1 and the periphery of the bottom of thecylindrical case 1.

Next, the details of the positive electrode 2, the separator 3, thenegative electrode 4 and the electrolyte will be explained.

(1) The Positive Electrode 2

This positive electrode 2 can be manufactured by adding a conductivematerial to an active material, i.e., nickel hydroxide powder, and theresultant mixture is kneaded together with a polymeric binder and waterto prepare a paste, which is then stuffed into an electroconductivesubstrate and, after being dried, molded into a predetermined shape.

As for the conductive material, cobalt oxide or cobalt hydroxide can beused.

Examples of polymeric binder are carboxymethyl cellulose, methylcellulose, sodium polyacrylate and polytetrafluoroethylene.

Examples of the electroconductive substrate are a metal net made ofnickel, stainless steel or stainless steel treated nickel plate, orsponge-like, fibrous or felt-like metallic porous body.

(2-1) The Negative Electrode

This negative electrode can be manufactured by adding a conductivematerial to a hydrogen-absorbing alloy powder, and the resultant mixtureis kneaded together with a polymeric binder and water to prepare apaste, which is then stuffed into an electroconductive substrate and,after being dried, molded into a predetermined shape.

Examples of the hydrogen-absorbing alloy are an alloy indicated by thefollowing items (1) to (5) may be employed.

(1) A hydrogen-absorbing alloy containing an alloy represented by any ofthe aforementioned general formulas (I), (II), (V) and (VI).

(2) A hydrogen-absorbing alloy which is featured in that a half-widthΔ(2θ) of at least one peak out of peaks of three strongest lines to beobtained by an X-ray diffraction using CuK_(α)-ray as a radiation sourcelies in the range of 0.2°≦Δ(2θ)≦50°, more preferably in the range of0.3°≦Δ(2θ)≦10°. This hydrogen-absorbing alloy should preferably be suchthat the size D of the crystallite thereof falls within the range of 0.8nm≦D≦50 nm.

(3) A hydrogen-absorbing alloy which is mechanically treated in vacuo orin an atmosphere of an inert gas or hydrogen.

(4) A hydrogen-absorbing alloy which is formed of a mixture comprisingan alloy having hydrogen-absorbing properties; and at least one additiveselected from the group consisting of (a) at least one element selectedfrom Group IA elements, Group IIA elements, Group IIIA elements, GroupIVA elements, VA elements, Group VIA elements, Group VIIA elements,Group VIIIA elements, Group IB elements, Group IIB elements, Group IIIBelements, Group IVB elements, Group VB elements and Group VIB elements;(b) an alloy formed of any combination of elements defined in said (a);and (c) an oxide of any of elements defined in said (a); said mixturebeing mechanically treated in vacuo or in an atmosphere of an inert gasor hydrogen.

(5) A hydrogen-absorbing alloy, which comprises an alloy havinghydrogen-absorbing properties; and 0.01 to 50% by volume of at least onepowdered additive having 0.01 to 100 μm in average diameter, which isdispersed in said alloy and selected from the group consisting of (a) atleast one element selected from Group IA elements, Group IIA elements,Group IIIA elements, Group IVA elements, VA elements, Group VIAelements, Group VIIA elements, Group VIIIA elements, Group IB elements,Group IIB elements, Group IIIB elements, Group IVB elements, Group VBelements and Group VIB elements; (b) an alloy formed of any combinationof elements defined in said (a); and (c) an oxide of any of elementsdefined in said (a).

Among the hydrogen-absorbing alloys or the alloys havinghydrogen-absorbing properties as referred in the above items (3) to (5),those containing an alloy represented by any of the aforementionedgeneral formulas (IV) to (VI) are preferable.

As for the polymeric binder, the same materials as employed for thepositive electrode ≦ can be used.

As for the conductive material, for example carbon black can be used.

Examples of the electroconductive substrate are a two-dimensionalsubstrate such as a punched metal, expanded metal, porous rigid plate, anickel net, and a three-dimensional substrate such as a felt-likemetallic porous body, sponge-like metallic substrate.

The negative electrode containing, as a raw material therefor, thehydrogen-absorbing alloy comprising an alloy represented by the generalformulas (I) and (II) is optimized regarding the content of magnesium inthe alloy, so that the reactivity thereof can be improved and at thesame time the deterioration resistance of the hydrogen-absorbing alloy,i.e., the stability of the hydrogen-absorbing alloy at the occasion ofhydrogen-absorption/desorption or charge/discharge cycle can beimproved. Further, with the provision of such a negative electrode, itis possible to produce an alkali secondary battery of excellent incapacity as well as in charge/discharge property.

The hydrogen-absorbing alloy containing an alloy represented by any ofthe aforementioned general formulas (V) and (VI) is featured in that itexhibits a prominent improvement on the hydrogen absorption properties,in particular an increase in amount of hydrogen absorption as comparedwith the conventional Mg₂Ni type alloy. This hydrogen-absorbing alloy isalso featured in that, as compared with the conventional rare earthelement type hydrogen-absorbing alloy, this hydrogen-absorbing alloy islarger in amount of hydrogen absorption per weight, cheaper inmanufacturing cost and lighter in weight. Therefore, an alkali secondarybattery provided with a negative electrode containing such ahydrogen-absorbing alloy as a negative electrode material is excellentin capacity as well as in charge/discharge property.

Further, an alkali secondary battery provided with a negative electrodecontaining a hydrogen-absorbing alloy represented by the aforementioneditems (2) to (5) as a negative electrode material would exhibit a stilllarger capacity and more excellent charge/discharge property.

(2-2) The Negative Electrode 4

This negative electrode 4 comprises a hydrogen-absorbing alloycontaining magnesium. When the negative electrode is immersed in a 6 to8N aqueous solution of an alkali hydroxide, (a) either the elution rateof magnesium ion into the aqueous solution of alkali hydroxide of normaltemperature is not more than 0.5 mg/kg alloy/hr, or the elution rate ofmagnesium ion into the aqueous solution of alkali hydroxide of 60° C. isnot more the 4 mg/kg alloy/hr, and (b) either the elution rate of acomponent element of alloy into the aqueous solution of alkali hydroxideof normal temperature is not more than 1.5 mg/kg alloy/hr, or theelution rate of a component element of alloy into the aqueous solutionof alkali hydroxide of 60° C. is not more than 20 mg/kg alloy/hr.

The inventors have established a method for evaluating the deteriorationrate of a hydrogen-absorbing alloy containing magnesium, and on thebasis of this method have found out a negative electrode comprising thehydrogen-absorbing alloy having a satisfactory reversibility andstability in the electrode reaction.

The elution rate of ion, in an aqueous solution of alkali hydroxide, ofa hydrogen-absorbing alloy, namely a corrosion rate is a parameterindicating a static stability of an alloy, so that it is generallyimpossible to estimate the stability of a dynamic cycle from only thisstatic stability. This is because the stability of cycle in ahydrogen-absorbing alloy is deemed to be much influenced by dynamicproperties such as an influence of hydrogen passing between the crystallattices of the alloy to the strain of crystal lattice in the process ofhydrogen-absorption and desorption, in addition to the static propertiesinherent to the alloy that can be determined by a chemical or physicalmodification through a contact with foreign additives or matters, orthrough a surface treatment.

In view of this, various kinds of negative electrodes (hydrogenelectrodes) were prepared by way of several methods using various kindsof hydrogen-absorbing alloys containing magnesium, each differing incomposition from each other and being treated with different methods,and evaluations of each negative electrode were performed. As a result,it has been found that, as far as a hydrogen-absorbing alloy having ahigh reversibility is concerned, a hydrogen-absorbing alloy meeting thefollowing conditions is excellent in stability.

Namely, a negative electrode should be characterized in that, when thenegative electrode is immersed in a 6 to 8N aqueous solution of analkali hydroxide, (a) as far as only magnesium is concerned, either theelution rate of magnesium ion into the aqueous solution of alkalihydroxide of normal temperature is not more than 0.5 mg/kg alloy/hr, orthe elution rate of magnesium ion into the aqueous solution of alkalihydroxide of 60° C. is not more than 4 mg/kg alloy/hr, and (b) as far asthe whole elements are concerned, either the elution rate of a componentelement of alloy into the aqueous solution of alkali hydroxide of normaltemperature is not more than 1.5 mg/kg alloy/hr, or the elution rate ofa component element of alloy into the aqueous solution of alkalihydroxide of 60° C. is not more than 20 mg/kg alloy/hr.

The reason for setting the elution rate at a temperature of 60° C. as acondition is as follows. Namely, in the case of hydrogen-absorbing alloywhich is practically useful, the ion elution rate at the normaltemperature of the alloy is as low as in the order of 0.5 mg/kg alloy/hrso that it is required for saving the measuring time and improving theaccuracy of measurement to accelerate the elution reaction rate.

Therefore, it is also possible to employ another set value representinga substantially the same degree of elution rate, which is to be measuredat a different temperature for evaluating the features of the negativeelectrode. However, the employment of high temperature exceeding over60° C. for the evaluation is not preferable, since it may give rise to aside reaction by flucturing an atmosphere temperature and fluctuation inmeasured values depending on the composition of the alloy and on thekinds of treatment.

Most convenient method of manufacturing the negative electrode is toemploy a hydrogen-absorbing alloy which is featured in that, when thehydrogen-absorbing alloy is immersed alone in a 6 to 10N aqueoussolution of an alkali hydroxide, the elution rate of magnesium ion intothe aqueous solution of alkali hydroxide of normal temperature is notmore than 0.5 mg/kg alloy/hr, or the elution rate of magnesium ion intothe aqueous solution of alkali hydroxide of 60° C. is not more than 4mg/kg alloy/hr, and the total of elution rate of component elements ofalloy into the aqueous solution of alkali hydroxide of normaltemperature is not more than 1.5 mg/kg alloy/hr, or the elution rate ofthe component elements into the aqueous solution of alkali hydroxide of60° C. is not more than 20 mg/kg alloy/hr.

(3) The Separator 3

The separator 3 may be formed of a nonwoven fabric made of a polymersuch as a polypropylene nonwoven fabric, a nylon nonwoven fabric or anonwoven fabric consisting of polypropylene fiber and nylon fiber. Inparticular, a polypropylene nonwoven fabric having its surface treatedinto hydrophilic nature is preferable as a separator.

(4) Alakali Electrolyte

Examples of the alkali electrolyte are an aqueous solution of sodiumhydroxide (NaOH), an aqueous solution of lithium hydroxide (LiOH), anaqueous solution of potassium hydroxide (KOH), a mixed solution ofsodium hydroxide (NaOH) and lithium hydroxide (LiOH), a mixed solutionof potassium hydroxide (KOH) and LiOH, and a mixed solution of NaOH, KOHand LIOH.

An alkali secondary battery according to the present invention comprisesa negative electrode accommodated in a case and including ahydrogen-absorbing alloy containing magnesium, a positive electrodeaccommodated in the case and so arranged as to opposite the negativeelectrode with a separator sandwiched therebetween, and an alkalielectrolyte filled therein, wherein a magnesium ion concentration in thealkali electrolyte 30 days or more after filling and sealing the alkalielectrolyte in the vessel is not more 2.2 mg/liter.

Reasons for defining the secondary battery by the ion concentration inthe electrolyte will be explained with reference to the case where it isused in an alkali secondary battery.

Generally, the amount of an electrolyte within an alkali secondarybattery is limited, so that even a small amount of ion elution into theelectrolyte will cause a substantial increase of ion concentration inthe electrolyte. Moreover, since the elution rate of ion decreases withthe increase in concentration of eluted ion in the electrolyte, the rateof increase in ion concentration in the electrolyte will be decreased toa negligible degree within a relatively short period of time. Further,in the case of a negative electrode (a hydrogen electrode) which is lowin deterioration rate, the elution rate of ion from the negativeelectrode is low from the beginning as explained with reference to thenegative electrode of this invention. In view of these facts, it can beassumed that the ion concentration of the electrolyte in the batteryafter 30 days as counted from the filling of the electrolyte is madesubstantially constant. Therefore, the ion concentration in anelectrolyte can be assumed as being one of the parameters as far as theelectrolyte within the battery is concerned.

As explained above, it is possible according to another embodiment ofthis invention to provide an alkali secondary battery with a negativeelectrode comprising the hydrogen-absorbing alloy having a satisfactoryreversibility and stability in the electrode reaction by establishing amethod for evaluating the deterioration rate of a hydrogen-absorbingalloy containing magnesium. Therefore, it is possible according to thisinvention to provide an alkali secondary battery of high capacity, inplace of the conventional alkali secondary battery (a nickel-cadmiumbattery, or nickel battery using LaNi₅ type hydrogen-absorbing alloy).

Meanwhile, the restriction of the amount of magnesium ion eluted into anelectrolyte in an alkali secondary battery is also effective inprohibiting the internal short through the formation of dendrite.

The present invention will be described in greater detail below by wayof its preferred examples.

EXAMPLES 1-5; AND COMPARATIVE EXAMPLES 1-6

Mg and Ni were dissolved in a high frequency furnace filled with anargon gas atmosphere, thereby preparing 11 kinds of hydrogen-absorbingalloy having a composition of Mg₂Ni_(y) (y is a value which is indicatedin Table 2 shown below).

The particles diameter of each of these 11 kinds of hydrogen-absorbingalloy thus obtained was adjusted to 45 to 75 μm, and then apredetermined amount of each of these alloys was dipped into an 8Naqueous solution of potassium hydroxide heated to 60° C. for 5 hours.Subsequently, the concentration of magnesium ion eluted into thisaqueous solution was measured. From this measurement, the relative valueof magnesium ion eluted and the concentration of magnesium ion elutedper mole of magnesium were calculated. The relative value of themagnesium eluted was calculated, setting the elution concentration fromthe pure magnesium to 100. The results are shown in Table 2 below.Further, a standardized data which was obtained by dividing the elutedamount by the ratio of magnesium in the alloy are shown in FIG. 4.

On the other hand, particles of each hydrogen-absorbing alloy pulverizedinto particles of 75 μm or less in diameter were charged into thepressure resistant vessel, and hydrogen gas was introduced into thisvessel under the conditions of 300° C. and 10 atm. Then, the amount ofhydrogen absorbed in the alloy was calculated from the decrease inpressure after 24 hours. The results are shown in Table 2 below.

TABLE 2 Eluted Amount of Mg₂Niy Mg concentration hydrogen compositionper molar adsorption Mg:Ni Relative ratio of (z in molar ratio y valueMg in alloy Mg₂Ni_(y)H_(z)) Comparative 100:0  0 100 1.00 0 Example 1Comparative 75:25 0.667 19 0.25 1.4 Example 2 Comparative 2:1 1.000 180.27 3.2 Example 3 Example 1 40:21 1.050 8 0.12 3.4 Example 2 64:361.125 7 0.11 3.5 Example 3 5:3 1.200 7 0.11 3.3 Example 4 8:5 1.250 80.13 3.0 Example 5 4:3 1.500 10 0.18 2.3 Comparative 8:7 1.750 105 1.970.7 Example 4 Comparative 50:50 2.000 145 2.90 0.4 Example 5 Comparative1:2 4.000 240 7.20 0.5 Example 6

As seen from Table 2, in the case of Mg₂Ni_(y), the amount of eluted Mgion indicated a smaller value than that of the pure Mg when the value ofy representing the amount of Ni was small. However, when the value of yexceeded over 1.5, the amount of eluted Mg ion was abruptly increased.In particular, when the value of y in the hydrogen-absorbing alloy wasin the range of 1<y≦1.5, the amount of eluted Mg ion was decreased.Further, the hydrogen-absorbing property of Mg₂Ni_(y) was notsubstantially altered when the value of y representing the amount of Niis around 1. However, when the value of y exceeded over 1.5, thehydrogen-absorbing property was abruptly deteriorated. In view of theseresults, it can seen that when the value of y in Mg₂Ni_(y) is in therange of 1<y≦1.5 as defined by this invention, a hydrogen-absorbingalloy having an excellent chemical stability and hydrogen-absorbingproperty can be obtained.

EXAMPLE 6 AND COMPARATIVE EXAMPLE 7

Mg and Ni were fused in a high frequency furnace filled with an argongas atmosphere, thereby preparing two kinds of hydrogen-absorbing alloy,each having a composition of Mg₂Ni_(1.5) (Example 6) and a compositionof Mg₂Ni_(0.84) (Comparative Example 7).

A block of each hydrogen-absorbing alloy was cut with a diamond cutterthereby preparing 5 kinds of band-like piece of alloy respectively.Since any flaw or irregularity formed on the surface of the band-likepiece of alloy may causes or promote a rupture of alloy, these testpieces were subjected to the following stress measurement afterpolishing the surfaces of these test pieces with a diamond paste 0.3 μmin particle size. The maximum stress of each band-like piece wasmeasured as shown in FIG. 5. Namely, a band-like piece 21 was supportedbetween a pair of supporting bars 22 arranged in parallel and kept apartby a distance of 20 mm. Then, a portion of the band-like piece 21corresponding to the center of the space between the pair of supportingbars 22 was pressed with a weight 23 to cause a bending of the band-likepiece 21 and at the same time the force required for bending theband-like piece 21 was measured, thus calculating the maximum stress onthe basis of the measurement.

The calculation of the maximum stress was made in accordance with theequation shown below. In this equation, the width of band-like alloypiece is indicated by w (mm); the thickness of band-like alloy piece, byT (mm); the distance between a pair of supporting bars was set to 20 mm,the force required for bending is defined as f/N, and the maximum stresswas indicated by σ. $\begin{matrix}{{\sigma \text{/}10^{6}\quad {N \cdot m^{- 2}}} = {{{moment}/{section}}\quad {modulus}}} \\{= {{\left( {20{f/4}} \right)/\left( {{Wt}^{2}/6} \right)} = {5f\text{/}6\quad W\quad t^{2}}}}\end{matrix}$

The maximum stress of each band-like alloy piece (test piece) measuredin this manner is shown in Table 3 below.

TABLE 3 Size of sample Modulus of Thickness Width Breaking Momentsection Max. stress (mm) (mm) load N 10³ Nm 10⁻⁹ m 10⁶ Nm⁻² Example 6Sample 1 0.49 10.1 2.05 10.2 0.41 25.2 Sample 2 0.50 9.9 2.83 14.2 0.4134.5 Sample 3 0.65 9.6 3.14 15.7 0.67 23.3 Sample 4 1.03 10.3 11.3 56.31.82 30.9 Sample 5 1.05 10.1 11.7 58.4 1.85 31.6 Comparative Sample 10.37 9.7 2.45 12.3 0.22 55.3 Example 7 Sample 2 0.60 10.6 6.20 31.0 0.6448.7 Sample 3 0.60 10.9 5.39 26.9 0.65 41.3 Sample 4 1.22 10.5 26.9 1342.60 51.8 Sample 5 1.30 10.3 27.7 138 2.92 47.3

As seen from this Table 3, a band-like alloy piece made of ahydrogen-absorbing alloy having a composition of Mg₂Ni_(1.5) (Example 6)indicated a low stress and an excellent workability (includingpulverization) as compared with a hydrogen-absorbing alloy having acomposition represented by Mg₂Ni_(0.84) (Comparative Example 7).

When the sectioned surface of the hydrogen-absorbing alloy of Example 6was investigated with a scanning electron microscope (SEM), it wasconfirmed from the resultant photograph that the sectioned surface wasmade up of a predominant portion of Mg₂Ni phase and a minor portion ofNi or other phases, and that there was a phase of high Mg content in asmall ratio on the grain boundary of Mg₂Ni phase. The same texture wasalso confirmed in the measurement of EPMA (electron probemicroanalyzer). Specifically, in the case of the hydrogen-absorbingalloy of Example 6, the area of the high Mg content phase was 8 to 9%based on the whole area, and the area occupied by Mg₂Ni phase and thephase consisting only of Ni was 90% or more. In a SEM photograph of thehydrogen-absorbing alloy of Comparative Example 7, most of the area wasoccupied by Mg₂Ni phase and the phase consisting only of Ni, and at thesame time a substantial amount of the high Mg content phase wasrecognized on the grain boundary of Mg₂Ni phase. Specifically, the areaof the high Mg content phase was 20% or more based on the whole area.

In view of these results of Example 6 and Comparative Example 7, it canbe seen that an alloy indicating a homogeneity (Mg₂Ni phase) of 90% ormore in a composition distribution as measured on the sectioned area thealloy is excellent in stability and mechanical pulverizability.

EXAMPLES 7-14 AND COMPARATIVE EXAMPLE 8-10

Eleven kinds of hydrogen-absorbing alloy blocks having a composition ofMg_(2−x)M2_(x)M1_(y), and value of y as shown in Table 4 shown below.

A plurality of band-like alloy pieces of the same size as the test piece21 of Example 6 were prepared from these hydrogen-absorbing alloyblocks. Then, the maximum stress of these test pieces was measured usingthe test apparatus shown in FIG. 5 and the calculating equation asmention above. The results obtained are shown in Table 4.

TABLE 4 Max. Hydrogen-absorbing Value stress alloy Mg_(2−x)M2_(x)M1y ofy 10⁶Nm⁻² Example 7 Mg₂Ni_(0.95)Fe_(0.1) 1.05 28.1 Example 8Mg₂Ni_(0.95)Co_(0.1) 1.05 28.6 Example 9 Mg₂Ni_(0.9)Cu_(0.2) 1.10 31.2Example 10 Mg_(1.9)Ca_(0.1)Ni_(1.1) 1.10 28.2 Example 11Mg_(1.9)La_(0.4)Ni_(1.1) 1.10 29.3 Example 12 Mg₂Ni_(0.9)Sn_(0.25) 1.1530.2 Example 13 Mg₂Ni₁Se_(0.1) 1.10 34.9 Example 14Mg_(1.9)Ca_(0.1)Ni_(0.9)Sn_(0.15) 1.05 30.8 Compara-Mg₂Ni_(0.95)Fe_(0.05) 1.00 44.8 tive Example 8 Compara-Mg_(1.9)Al_(0.1)Ni_(0.9) 0.90 54.3 tive Example 9 Compara-Mg₂Ni_(0.45)Sn_(0.45) 0.90 53.3 tive Example 10

As apparent from Table 4, in the case of Mg_(2−x)M2_(x)M1_(y), a maximumstress of about 30×10⁶ Nm⁻² was obtainable when the value of y is over 1as shown in Example 7 to 14. By contrast, in the case of ComparativeExamples 8 to 10 where the value of y is not more than 1, the maximumstress thereof was about 50×10⁶ Nm⁻², indicating that a force of severalten % higher than those of Examples 7 to 14 was required in theseComparative Examples 8 to 10.

EXAMPLE 15

A hydrogen-absorbing alloy comprising predetermined amount of Mg and Niand having a composition of Mg₂Ni_(y) was prepared by way of anannealing method. Then, the alloy was sealed in a quartz tube filledwith an argon atmosphere, and slowly annealed for about one month at atemperature of 500° C. thereby obtaining a hydrogen-absorbing alloycomprising a composition of Mg₂Ni_(1.01).

The hydrogen-absorbing alloy thus obtained was dipped in an aqueoussolution of alkali hydroxide, and the amount of magnesium ion eluted wasmeasured. As a result, the relative amount of magnesium ion eluted wasfound to be 9, the value obtained by dividing the amount by 66.4%, i.e.,the magnesium content in the alloy was found to be 0.14, indicating anexcellent chemical stability.

FIG. 6 illustrates a temperature scanning typehydrogen-absorption/desorption-evaluating apparatus employed in theevaluation of hydrogen-absorbing alloys obtained in Examples beginningfrom Example 16. Referring to FIG. 16, a hydrogen gas cylinder 31 isconnected via a pipe 32 to a test sample vessel 33. The middle portionof the pipe 32 is branched, and a distal end of the branched pipe 34 isconnected to a vacuum pump 35. A manometer 36 is mounted on a pipeportion branched from the branched pipe 34. On the pipe 32 interposedbetween the hydrogen gas cylinder 31 and the test sample vessel 33 aremounted in the order (starting from the hydrogen gas cylinder 31) of afirst valve 37 ₁ and a second valve 37 ₂. On a portion of the pipe 32disposed between the first valve 37 ₁ and a second valve 37 ₂ isconnected a pressure accumulator 38. Further, a third valve 37 ₃ ismounted on a portion of the branched pipe 34 interposed between thevacuum pump 35 and the manometer 36. The test sample vessel 33 isprovided with a heater 39. A thermocouple 40 is disposed inside the testsample vessel 33. A temperature controller 42 to be controlled by acomputer 41 is connected to both of the thermocouple 40 and the heater39 so as to control the temperature of the heater 39 on the basis of adetected temperature from the thermocouple 40. A recorder 43 to becontrolled by the computer 41 is connected to both of the manometer 36and the temperature controller 42.

EXAMPLES 16 AND 17 AND COMPARATIVE EXAMPLES 11 AND 12

Various kinds of hydrogen-absorbing alloys, each having a composition ofMg_(2−x)M2_(x)M1_(y), but differing in components of Mg_(2−x)M2_(x) andM₁, and values of x and y, i.e., an alloy of Mg_(1.9)Al_(0.1)Ni_(1.05)(M1=Ni, M2=Al, x=0.1, y=1.05; Example 16), an alloy ofMg_(1.9)Al_(0.1)Ni (M1=Ni, M2=Al, x=0.1, y=1; Comparative Example 11),an alloy of Mg_(1.9)Mn_(0.1)Ni_(1.05) (M1=Ni, M2=Mn, x=0.1, y=1.05;Example 17), an alloy of Mg_(1.9)Mn_(0.1)Ni (M1=1, M2=Mn, x=0.1, y=1.0;Comparative Example 12) and Mg₂Ni (Comparative Example 3) were prepared.

Then, each of these hydrogen-absorbing alloys was charged into the testsample vessel 33. Thereafter, the first valve 37 ₁ was closed, and bothof the second valve 37 ₂ and the third valve 37 ₃ were opened. Underthis condition, the vacuum pump 35 was actuated to exhaust the air ineach of the pipe 32, the branched pipe 34, the pressure accumulator 38and the test sample vessel 33. Then, after closing the second valve 37 ₂and the third valve 37 ₃, the first valve 37 ₁ was opened to supplyhydrogen from the hydrogen gas cylinder 31 to each of the pipe 32, thebranched pipe 34, the pressure accumulator 38 and the test sample vessel33 thereby carrying out a hydrogen displacement of them. Subsequently,the first valve 37 ₁ was closed and at the same time, the amount ofhydrogen introduced was calculated from the pressures indicated by themanometer 36. Thereafter, the second valve 37 ₂ was opened therebyfeeding hydrogen to the test sample vessel 33, and the temperaturethereof was monitored with the thermocouple 40. Then, the temperature ofthe test sample vessel 33 was allowed to raise at a constant rate bycontrolling the thermocouple 40 and the temperature controller 42. Atthe same time, the temperature of the test sample vessel 33 was scannedby using the heater 39 receiving this control signal. At this moment, achange in pressure if any within the test sample vessel 33 was detectedby means of the manometer 36 and recorded in the recorder 43. Thepressure change (a decrease in temperature resulting from the hydrogenabsorption by the hydrogen-absorbing alloy) due to the rise intemperature of the test sample vessel 33 is shown in FIG. 7.

As seen from FIG. 7, a hydrogen-absorbing alloy ofMg_(1.9)Al_(0.1)Ni_(1.05) (Example 16) is capable of absorbing hydrogenat a lower temperature as compared with a hydrogen-absorbing alloy ofMg_(1.9)Al_(0.1)Ni (Comparative Example 11). A hydrogen-absorbing alloyof Mg_(0.9)Mn_(0.1)Ni_(1.05) (Example 17) is also capable of absorbinghydrogen at a lower temperature as compared with a hydrogen-absorbingalloy of Mg_(1.9)Mn_(0.1)Ni (Comparative Example 12). In particular, thehydrogen-absorbing alloy of Example 16 wherein M2 is substituted by Alis seen to be more suited, as compared with the hydrogen-absorbing alloyof Example 17 wherein M2 is substituted by Mn, in lowering thehydrogen-absorbing temperature or the temperature suited for hydrogenabsorption. Further, it can be seen that the hydrogen-absorbing alloysof Examples 16 and 17 have an excellent hydrogen-absorbing capacitywhich is comparable to that of the alloy of Mg₂Ni (Comparative Example3). Accordingly, it can be seen that when part of Mg is substituted withM2 (Al or Mn), it is possible to lower the temperature suited forhydrogen-absorption, while maintaining an excellent hydrogen-absorptioncapacity.

In another test, the relationship between the concentration of absorbedhydrogen and the hydrogen-absorption temperature was investigated, usingthe hydrogen-absorbing alloys of Mg_(1.9)Al_(0.1)Ni_(1.05) (Example 16),Mg_(1.9)Mn_(0.1)Ni_(1.05) (Example 17), Mg_(1.9)Al_(0.1)Ni (ComparativeExample 11) and Mg₂Ni (Comparative Example 3), thereby investigating thetemperature requiring to absorb hydrogen up to H/M=0.1 (this means thatthe ratio of the number of atoms absorbed to the number of atoms of ahydrogen-absorbing alloy is 0.1). Furthermore, the relative value of theconcentration of eluted magnesium and the concentration of elutedmagnesium per mole of Mg of the alloy were measured using thehydrogen-absorbing alloys of Examples 16 and 17, and ComparativeExamples of 11 and 3 in the same manner as explained in Example 1. Inthis case, the relative value of the eluted magnesium ion concentrationwas calculated by setting the magnesium ion concentration eluted fromthe pure magnesium as being 100. The results obtained are shown in FIG.5.

TABLE 5 Eluted Mg concentration per molar Hydrogen-absorbing ratio ofTem- alloy Value Relative Mg in perature Mg_(2−x)M2_(x)M1_(y) of y valuealloy (° C.) Example 16 Mg_(1.9)Al_(0.1)Ni_(1.05) 1.05 7 0.11 70Comparative Mg_(1.9)Al_(0.1)Ni 1.00 17 0.27 75 Example 11 Example 17Mg_(1.9)Ni_(1.06)Mn_(0.1) 1.16 7 0.12 110 Comparative Mg₂Ni 1.00 18 0.27140 Example 3

As apparent from Table 5, with the employment of a hydrogen-absorbingalloy comprising Mg substituted partially by M2 (Al or Mn) and M1 whosevalue of y exceeding over 1, it is possible to realize the lowering ofhydrogen-absorbing temperature and to improve the chemical stability ofthe alloy.

EXAMPLE 18

A hydrogen-absorbing alloy having a composition ofMg_(1.9)Al_(0.1)Ni_(0.55)Co_(0.55) (M1=Ni and Co, M2=Al, x=0.1, y=1.10in Mg_(2−x)M2_(x)M1_(y) was prepared. Then, the pressure change (adecrease in temperature resulting from the hydrogen absorption by thehydrogen-absorbing alloy) due to the rise in temperature of the testsample vessel was measured in the same manner as in Example 16 using theshown in FIG. 6. As result, a graph showing the characteristics of thealloy as shown in FIG. 8 was obtained. The results obtained from thehydrogen-absorbing alloy of Mg₂Ni (Comparative Example 3) are also shownin FIG. 8.

As shown in FIG. 8, the hydrogen-absorbing alloy wherein M1 issubstituted by Ni and Co is capable of lowering the temperature suitedfor hydrogen-absorption, while maintaining an excellenthydrogen-absorption capacity.

In another test, the relative value of the concentration of elutedmagnesium and the concentration of eluted magnesium per mole of Mg ofthe alloy of this Example were measured in the same manner as explainedin Example 1. In this case, the relative value of the eluted magnesiumion concentration was calculated by setting the magnesium ionconcentration eluted from the pure magnesium as being 100. As a result,the relative value of the concentration of eluted magnesium was found tobe 8, and the concentration of eluted magnesium per mole of Mg of thealloy was found to be 0.13.

EXAMPLES 19 AND 20, AND COMPARATIVE EXAMPLE 13

Various kinds of hydrogen-absorbing alloys, each having a composition ofM_(2−x)M2_(x)M1_(y), but differing in components of M_(2−x)M2_(x) andM1, and values of x and y, i.e., an alloy of Zr_(1.9)V_(0.1)Fe_(1.05)(M=Zr, M1=Fe, M2=V, x=0.1, y=1.05; Example 19), an alloy ofZr_(1.9)Cr_(0.1)Fe_(1.05) (M=Zr, M1=Fe, M2=Cr, x=0.1, y=1.05; Example20), and Zr₂Fe (Comparative Example 13) were prepared. Then, thepressure change (a decrease in temperature resulting from the hydrogenabsorption by the hydrogen-absorbing alloy) due to the rise intemperature of the test sample vessel was measured in the same manner asin Example 16 using the shown in FIG. 6. At the same time, aninvestigation was also conducted regarding the temperature requiring toabsorb hydrogen up to H/M=0.1 (this means that the ration of the numberof atoms absorbed to the number of atoms of a hydrogen-absorbing alloyis 0.1). Results are shown in Table 6 below.

TABLE 6 Hydrogen-absorbing Tempe- alloy Value rature M_(2−x)M2_(x)M1_(y)of y (° C.) Example 19 Zr_(1.9)V_(0.1)Fe_(1.05) 1.16 340 Example 20Zr_(1.9)Cr_(0.1)Fe_(1.05) 1.16 295 Compara- Zr₂Fe 1.00 380 tive Example13

As apparent from Table 6, with the employment of a hydrogen-absorbingalloy comprising Zr as M, which is substituted partially by M2 (V andCr), and M1 whose value of y exceeding over 1 (Examples 19 and 20), itis possible to realize the lowering of hydrogen-absorbing temperature.

EXAMPLES 21-26 AND COMPARATIVE EXAMPLES 3, 14 AND 15

First, Mg, Ni, Ag, Cd, Ca, Pd, Al, In, Co and Ti where dissolved in ahigh frequency furnace filled with an argon gas atmosphere, therebypreparing 9 kinds of hydrogen-absorbing alloy, each having a compositionof Mg_(2−x)M2_(x)M1_(y) as shown in Table 7.

Then, each of these hydrogen-absorbing alloys was charged into the testsample vessel 33 as a test sample. Thereafter, the first valve 37 ₁ wasclosed, and both of the second valve 37 ₂ and the third valve 37 ₃ wereopened. Under this condition, the vacuum pump 35 was actuated to exhaustthe air in each of the pipe 32, the branched pipe 34, the pressureaccumulator 38 and the test sample vessel 33. Then, after closing thesecond valve 37 ₂ and the third valve 37 ₃, the first valve 37 ₁ wasopened to supply hydrogen from the hydrogen gas cylinder 31 to each ofthe pipe 32, the branched pipe 34, the pressure accumulator 38 and thetest sample vessel 33 thereby carrying out a hydrogen displacement ofthem. Subsequently, the first valve 37 ₁ was closed and the second valve37 ₂ was opened thereby feeding hydrogen to the test sample vessel 33,and the pressures and temperatures recorded in the recorder 43 werechecked.

In the case of Examples 21 to 26, and Comparative Examples 3, 14 and 15the pressure of the hydrogen gas cylinder 31 was preset at the moment ofthe hydrogen displacement so as to control the pressure (initialpressure) in the vessel 33 to be kept at about 10 atm., and themeasurement-initiating temperature was set to room temperature (about25° C.).

Subsequently, the temperature within the test sample vessel 33 wascontrolled to be raised at a rate of 0.5° C. per minute by thecontrolling of the computer 41 and the temperature controller 42. At thesame time, the temperature of the test sample vessel 33 was scanned byusing the heater 39 receiving this control signal. At this moment,changes in pressure and temperature if any within the test sample vessel33 were detected by means of the manometer 36 and recorded in therecorder 43.

On the other hand, the change in pressure within the test sample vessel33, resulting from the rise in temperature in the above operation wasmonitored, and, on the basis of this pressure decrease, the temperaturewhen H/M=0.1 was reached (i.e., when the number of hydrogen atomsabsorbed per one mole of atom of the alloy was reached to 0.1) wasdetermined, assuming this temperature as a standard of the minimumtemperature which enables the hydrogen-absorbing alloy to perform ahydrogen-absorbing reaction. Further, the relative value of theconcentration of eluted magnesium and the concentration of elutedmagnesium per mole of Mg of the alloy of Examples 21 to 26 andComparative Examples 3, 14 and 15 were measured in the same manner asexplained in Example 1. In this case, the relative value of the elutedmagnesium ion concentration was calculated by setting the magnesium ionconcentration eluted from the pure magnesium as being 100. Results areshown in Table 7 below.

TABLE 7 Eluted Mg concentration per molar Hydrogen-absorbing Value ratioTem- alloy of y Relative of Mg perature Mg_(2−x)M2_(x)M1_(y) (° C.)value in alloy (° C.) Example 21 Mg₂Ag_(0.22)Ni_(1.11) 1.11 7 0.11 120Comparative Mg_(1.9)Al_(0.1)Ni 1.00 17 0.27 75 Example 14 Example 22Mg₂Co_(1.24)In_(0.35) 1.59 8 0.14 110 Example 23 Mg₂Co_(1.11)In_(0.11)1.22 9 0.15 150 Example 24 Mg_(1.5)Ca_(0.5)Ni_(1.5) 2.00 7 0.19 115Ag_(0.5) Example 25 Mg_(1.76)Ca_(0.5)Ni_(1.5) 1.88 7 0.15 110 Ag_(0.38)Example 26 Mg₂Ni_(1.25)In_(0.25) 1.75 6 0.11 125 W_(0.25) ComparativeMg₂Ni 1.00 18 0.27 140 Example 3 Comparative Mg₂Co 1.00 58 0.87 170Example 15

As apparent from Table 7, with the employment of a hydrogen-absorbingalloy according to Examples 21 to 26, it is possible to realize thelowering of hydrogen-absorbing temperature and to improve the chemicalstability of the alloy as compared with those of Comparative Examples 3,14 and 15.

EXAMPLES 27-38 AND COMPARATIVE EXAMPLE 16

Thirteen kinds of hydrogen-absorbing alloy as shown in the followingTable 8, each having the general formula (V) of(Mg_(1−x)M3_(x))_(20−y)M4 (wherein x is defined as 0<x<0.5; and y isdefined as 0≦y<18), were charged into the test sample vessel 33.Thereafter, the first valve 37 ₁ was closed, and both of the secondvalve 37 ₂ and the third valve 37 ₃ were opened. Under this condition,the vacuum pump 35 was actuated to exhaust the air in each of the pipe32, the branched pipe 34, the pressure accumulator 38 and the testsample vessel 33. Then, after closing the second valve 37 ₂ and thethird valve 37 ₃, the first valve 37 ₁ was opened to supply hydrogenfrom the hydrogen gas cylinder 31 to each of the pipe 32, the branchedpipe 34, the pressure accumulator 38 and the test sample vessel 33thereby carrying out a hydrogen displacement of them. Subsequently, thefirst valve 37 ₁ was closed and the amount of hydrogen thus introducedup to this moment was calculated from the pressure of the system whichwas indicated by the manometer 36. Then, the second valve 37 ₂ wasopened thereby feeding hydrogen to the test sample vessel 33, and thetemperature therein was monitored by making use of the thermocouple 40.At this moment, the thermocouple 40 and the temperature controller 42were controlled so as to keep the temperature inside the test samplevessel 33 constant. In this case, changes in pressure if any within thetest sample vessel 33 were detected by means of the manometer 36 andrecorded in the recorder 43.

By making use of the aforementioned evaluation apparatus, the rate ofhydrogen absorption by each hydrogen-absorbing alloy at a temperature of100° C. was measured. This hydrogen absorption rate was indicated by theamount of hydrogen (wt. %) which was absorbed in the hydrogen-absorbingalloy during a time period of one hour starting from the introduction ofa predetermined amount of hydrogen into the sample vessel. Results areshown in Table 8 below.

TABLE 8 Hydrogen absorp- tion Hydrogen-absorbing alloy rate Compara-Mg₄Ni 1.0 tive Example 16 Example (Mg_(0.5)V_(0.5))₂₀Zn_(0.3)Ni_(0.7)7.2 27 Example (Mg_(0.85)Mn_(0.15))₁₇Ni_(0.9)Cu_(0.1) 6.6 28 Example(Mg_(0.8)S_(0.2))_(7.8)Ni 4.7 29 Example(Mg_(0.7)C_(0.3))₆Ni_(0.5)Co_(0.5) 4.5 30 Example(Mg_(0.6)Ru_(0.4))₄Ni_(0.3)Fe_(0.7) 4.0 31 Example(Mg_(0.9)Pt_(0.1))₅Si_(0.2)Ni_(0.8) 4.2 32 Example (Mg_(0.5)Pd_(0.5))₈Cu5.0 33 Example (Mg_(0.8)Au_(0.1)Al_(0.1))₁₄Ni 6.2 34 Example(Mg_(0.99)Mn_(0.01))_(7.8)Ni_(0.8)Fe_(0.2) 4.9 35 Example(Mg_(0.7)Ti_(0.3))₁₇Ni_(0.3)Co_(0.7) 6.5 36 Example(Mg_(0.9)Nb_(0.1))₁₀Ni 5.3 37 Example(Mg_(0.8)Ag_(0.2))₈Ni_(0.8)Fe_(0.2) 5.0 38

As apparent from Table 8, with the employment of hydrogen-absorbingalloys according to Examples 27 to 38 each containing an increasedamount of Mg as compared with the hydrogen-absorbing alloy ofComparative Example 16, it was possible to prominently increase theamount of hydrogen absorption and to improve the hydrogen absorbingproperties of the alloy.

EXAMPLES 39-50, AND COMPARATIVE EXAMPLE 17

First of all, thirteen kinds of powdered hydrogen-absorbing alloy asshown in the following Table 9, each having the general formula (V) of(Mg_(1−x)M3_(x))_(20−y)M4 (wherein x is defined as 0<x<0.5; and y isdefined as 0≦y<18), were respectively mixed with electrolytic copperpowder in the weight ratio of 1:1, and 1 g of the resultant mixture wassubjected to compression for 5 minutes by applying a pressure of 10,000kg in a tablet-molding device (inner diameter: 10 mm) thereby obtaininga pellet. This pellet was then sandwiched between Ni wire nettings, andthe peripheral portion thereof was spot-welded and pressed.Subsequently, to this pressed body was connected a Ni lead wire by meansof spot-welding thereby preparing thirteen different kinds ofhydrogen-absorbing alloy electrodes (negative electrodes).

The hydrogen electrodes thus obtained were respectively dipped, togetherwith a sintered nickel electrode constituting a counter electrode, intoa 8N aqueous solution of potassium hydroxide, and then, acharge/discharge cycle test was performed at a temperature of 25° C. Inthis charge/discharge cycle test, each cycle was consisted of steps,i.e. the charging was conducted under the condition of 100 mA per 1 g ofhydrogen-absorbing alloy for 10 hours, and, after a ten minutecessation, the discharge was conducted under the condition of 20 mA per1 g of hydrogen-absorbing alloy until the voltage against the mercuryoxide electrode was lowered down to −0.5 V. This charge/discharge cyclewas repeated to obtain a maximum discharge capacity of each negativeelectrode. The results of this cycle test were shown in Table 9 below.

TABLE 9 Discharge capacity Hydrogen-absorbing alloy (mAh/g) Compara-Mg_(3.5)Ni 15 tive Example 17 Example(Mg_(0.8)Ta_(0.2))_(7.8)Cu_(0.4)Ni_(0.6) 410 39 Example(Mg_(0.6)Os_(0.4))₉Ni_(0.5)Cu_(0.5) 420 40 Example(Mg_(0.7)Re_(0.3))₁₅Si_(0.4)Ni_(0.6) 710 41 Example(Mg_(0.98)Ir_(0.02))₁₀Ni_(0.6)Co_(0.4) 425 42 Example(Mg_(0.8)Rh_(0.2))₈Ni 405 43 Example(Mg_(0.97)C_(0.03))₃Ni_(0.2)Fe_(0.8) 120 44 Example(Mg_(0.9)Ag_(0.1))₁₄Cu 640 45 Example (Mg_(0.5)Al_(0.5))_(7.8)Ni 420 46Example (Mg_(0.94)P_(0.06))₁₀Ni_(0.6)Co_(0.4) 415 47 Example(Mg_(0.9)In_(0.1))₈Ni 390 48 Example (Mg_(0.8)Pt_(0.2))₅Ni 180 49Example (Mg_(0.8)Au_(0.2))₃Ni_(0.2)Fe_(0.8) 135 50

As apparent from Table 9, the negative electrodes (Examples 39 to 50)each containing a hydrogen-absorbing alloy represented by the generalformula (V) of (Mg_(1−x)M3_(x))_(20−y)M4 and containing an increasedamount of Mg as compared with the hydrogen-absorbing alloy ofComparative Example 17 were effective in increasing the amount ofhydrogen absorption and in prominently improving the hydrogen absorbingproperties of the alloy.

EXAMPLES 51-60, AND COMPARATIVE EXAMPLE 18

Eleven kinds of hydrogen-absorbing alloy as shown in the following Table10, each having the general formula (V) of (Mg_(1−x)M3_(x))_(20−y)M4(wherein x is defined as 0<x<0.5; and y is defined as 0≦y<18), wererespectively measured of the amount of hydrogen absorption at atemperature of 25° C. by making use of the aforementionedhydrogen-absorption/desorption property-evaluating apparatus shown inFIG. 6. In this case, the amount of hydrogen absorption was indicated bythe amount of hydrogen (wt. %) which was absorbed in thehydrogen-absorbing alloy during a time period of 20 hours starting fromthe introduction of a predetermined amount of hydrogen into the samplevessel. Results are shown in Table 10 below.

TABLE 10 Amount of hydrogen absorp- tion Hydrogen-absorbing alloy (wt %)Compara- Mg_(3.2)Ni 0.5 tive Example 18 Example(Mg_(0.9)Y_(0.1))_(7.8)Zn_(0.1)Ni_(0.9) 4.8 51 Example(Mg_(0.7)Sc_(0.3))₅Cu_(0.8)Ni_(0.2) 4.2 52 Example(Mg_(0.6)La_(0.4))₃Ni_(0.5)Co_(0.5) 3.1 53 Example(Mg_(0.6)Hf_(0.2)Pt_(0.2))₄Fe 4.0 54 Example(Mg_(0.5)Zr_(0.5))₁₀Cu_(0.5)Ni_(0.5) 5.3 55 Example(Mg_(0.8)Pb_(0.2))₅Ni 4.2 56 Example (Mg_(0.9)Y_(0.1))₈Sn_(0.4)Ni_(0.6)5.0 57 Example (Mg_(0.4)In_(0.4)W_(0.2))_(7.8)Ni 4.8 58 Example(Mg_(0.7)La_(0.3))₁₇Cu_(0.3)Ni_(0.7) 6.6 59 Example(Mg_(0.9)Tl_(0.1))₉Si_(0.05)Co_(0.95) 5.0 60

As apparent from Table 10, with the employment of hydrogen-absorbingalloys according to Examples 51 to 60 each containing an increasedamount of Mg as compared with the hydrogen-absorbing alloy ofComparative Example 18, it was possible to prominently increase theamount of hydrogen absorption and to improve the hydrogen absorbingproperties of the alloy.

EXAMPLES 61-71, AND COMPARATIVE EXAMPLE 19

First of all, twelve kinds of powdered hydrogen-absorbing alloy as shownin the following Table 11, each having the general formula (V) of(Mg_(1−x)M3_(x))_(20−y)M4 (wherein x is defined as 0<x<0.5; and y isdefined as 0≦y<18), were respectively mixed with electrolytic copperpowder in the weight ratio of 1:1, and 1 g of the resultant mixture wassubjected to compression for 5 minutes by applying a pressure of 10,000kg in a tablet-molding device (inner diameter: 10 mm) thereby obtaininga pellet. This pellet was then sandwiched between Ni wire nettings, andthe peripheral portion thereof was spot-welded and pressed.Subsequently, to this pressed body was connected a Ni lead wire by meansof spot-welding thereby preparing twelve different kinds ofhydrogen-absorbing alloy electrodes (negative electrodes).

The hydrogen electrodes thus obtained were respectively dipped, togetherwith a sintered nickel electrode constituting a counter electrode, intoa 8N aqueous solution of potassium hydroxide, and then, acharge/discharge cycle test was performed at a temperature of 25° C. Inthis charge/discharge cycle test, each cycle was consisted of steps,i.e. the charging was conducted under the condition of 100 mA per 1 g ofhydrogen-absorbing alloy for 10 hours, and, after a ten minutecessation, the discharge was conducted under the condition of 20 mA per1 g of hydrogen-absorbing alloy until the voltage against the mercuryoxide electrode was lowered down to −0.5 V. This charge/discharge cyclewas repeated to obtain a maximum discharge capacity of each negativeelectrode. The results of this cycle test were shown in Table 11 below.

TABLE 11 Discharge capacity Hydrogen-absorbing alloy (mAh/g) Compara-Mg_(4.0)Ni 25 tive Example 19 Example(Mg_(0.9)Ce_(0.1))₈Zn_(0.3)Ni_(0.7) 420 61 Example(Mg_(0.85)La_(0.05)C_(0.1))₁₃Fe 510 62 Example(Mg_(0.7)Pr_(0.3))_(7.8)Ni 410 63 Example(Mg_(0.25)Zr_(0.4)Mo_(0.35))₃Ni 125 64 Example(Mg_(0.8)Sm_(0.2))₅Si_(0.3)Cu_(0.7) 200 65 Example(Mg_(0.5)Y_(0.4)Al_(0.1))₄Co 180 66 Example(Mg_(0.9)Zr_(0.1))₉Si_(0.2)Ni_(0.8) 390 67 Example(Mg_(0.8)In_(0.2))_(7.8)Zn_(0.4)Cu_(0.6) 370 68 Example(Mg_(0.99)Hf_(0.01))₃Sn_(0.2)Ni_(0.8) 110 69 Example(Mg_(0.8)Hf_(0.2))₄Ni 175 70 Example (Mg_(0.8)Y_(0.2))₈Cu_(0.5)Ni_(0.5)390 71

As apparent from Table 11, the negative electrodes (Examples 61 to 71)each containing a hydrogen-absorbing alloy represented by the generalformula (V) of (Mg_(1−x)M3_(x))_(20−y)M4 and containing an increasedamount of Mg as compared with the hydrogen-absorbing alloy ofComparative Example 19 were effective in increasing the amount ofhydrogen absorption and in prominently improving the hydrogen absorbingproperties of the alloy.

EXAMPLES 72-74

Three kinds of hydrogen-absorbing alloy as shown in the following Table12, each having the general formula (VI) of (Mg_(1−x)M5_(x))_(20−y)M6(wherein x is defined as 0<x<0.5; and y is defined as 0≦y<18), wererespectively measured of the amount of hydrogen absorption at atemperature of 25° C. by making use of the aforementionedhydrogen-absorption/desorption property-evaluating apparatus shown inFIG. 6. In this case, the amount of hydrogen absorption was indicated bythe amount of hydrogen (wt. %) which was absorbed in thehydrogen-absorbing alloy during a time period of 20 hours starting fromthe introduction of a predetermined amount of hydrogen into the samplevessel. Results are shown in Table 12.

Furthermore, the aforementioned powdered hydrogen-absorbing alloys wererespectively mixed with electrolytic copper powder in the weight ratioof 1:1, and 1 g of the resultant mixture was subjected to compressionfor 5 minutes by applying a pressure of 10,000 kg in a tablet-moldingdevice (inner diameter: 10 mm) thereby obtaining a pellet. This pelletwas then sandwiched between Ni wire nettings, and the peripheral portionthereof was spot-welded and pressed. Subsequently, to this pressed bodywas connected a Ni lead wire by means of spot-welding thereby preparingthree different kinds of hydrogen-absorbing alloy electrodes (negativeelectrodes).

The hydrogen electrodes thus obtained were respectively dipped, togetherwith a sintered nickel electrode constituting a counter electrode, intoa 8N aqueous solution of potassium hydroxide, and then, acharge/discharge cycle test was performed at a temperature of 25° C. Inthis charge/discharge cycle test, each cycle was consisted of steps,i.e. the charging was conducted under the condition of 100 mA per 1 g ofhydrogen-absorbing alloy for 10 hours, and, after a ten minutecessation, the discharge was conducted under the condition of 20 mA per1 g of hydrogen-absorbing alloy until the voltage against the mercuryoxide electrode was lowered down to −0.5 V. This charge/discharge cyclewas repeated to obtain a maximum discharge capacity of each negativeelectrode. The results of this cycle test were shown in Table 12 below.

TABLE 12 Amount of hydrogen absorption Discharge Hydrogen-absorbingalloy (wt %) capacity (mAh/g) Example 72(Mg_(0.85)Ca_(0.15))₁₆Ni_(0.8)Cu_(0.2) 6.4 650 Example 73(Mg_(0.7)Ca_(0.3))₁₅Fe_(0.3)Co_(0.7) 6.0 720 Example 74(Mg_(0.8)Sr_(0.2))₁₁Co_(0.05)Cu_(0.95) 5.5 580

As apparent from Table 12, the hydrogen-absorbing alloys according toExamples 72 to 74, where the Mg site in the alloy was substituted by M5or an element which has 1 to 1.5 times as high atomic radius as that ofMg (excluding elements which are more electro-negative than Mg), andwhere at least one element selected from Ni, Fe, Co, Cu, Zn, Sn and Siwas employed as M6 exhibited a prominent increase in hydrogen absorptionand an improved hydrogen absorbing properties.

It can be also understood that negative electrodes containing theaforementioned hydrogen absorbing alloy are capable of increasing thedischarge capacity and prominently improving the charge/dischargecharacteristics thereof, respectively.

EXAMPLE 75

A hydrogen-absorbing alloy comprising Mg₂Ni was introduced into a roundbottom flask provided therein with a rotator, and attached thereto witha dropping funnel and a cooling pipe. Then, the interior of the flaskwas evacuated by means of a vacuum pump and, after being head-dried witha heating gun, displaced by argon gas. Subsequently, THF was introducedinto the flask while allowing the argon gas to flow into the flask, and1-bromo-3-ethane was slowly dripped under stirring into the flask viathe dropping funnel, thereby allowing the hydrogen-absorbing alloy toreact with 1-bromo-3-ethane. Upon finishing the dripping, the stirringwas suspended thereby allowing the hydrogen-absorbing alloy to settle,and then this settled hydrogen-absorbing alloy was filtered to obtain asurface-modified hydrogen-absorbing alloy.

This surface-modified hydrogen-absorbing alloy as well as ahydrogen-absorbing alloy which was not surface-modified wereinvestigated of their hydrogen-absorbing properties using aforementionedhydrogen-absorption/desorption property-evaluating apparatus shown inFIG. 6. This measurement was conducted by monitoring the pressure changein the reaction vessel when a predetermined volume of hydrogen wasintroduced into the reaction vessel.

Specifically, first of all, each of these hydrogen-absorbing alloy s wascharged into the test sample vessel 33. Thereafter, the first valve 37 ₁was closed, and both of the second valve 37 ₂ and the third valve 37 ₃were opened. Under this condition, the vacuum pump 35 was actuated toexhaust the air in each of the pipe 32, the branched pipe 34, thepressure accumulator 38 and the test sample vessel 33. Then, afterclosing the second valve 37 ₂ and the third valve 37 ₃, the first valve37 ₁ was opened to supply hydrogen from the hydrogen gas cylinder 31 toeach of the pipe 32, the branched pipe 34, the pressure accumulator 38and the test sample vessel 33 thereby carrying out a hydrogendisplacement of them. Subsequently, the first valve 37 ₁ was closed andat the same time, the amount of hydrogen introduced was calculated fromthe pressures indicated by the manometer 36. Thereafter, the secondvalve 37 ₂ was opened thereby feeding hydrogen to the test sample vessel33, and the temperature thereof was monitored with the thermocouple 40.At this moment, the thermocouple 40 and the temperature controller 42were controlled so as to keep the temperature inside the test samplevessel 33 constant. At this moment, a change in pressure if any withinthe test sample vessel 33 was detected by means of the manometer 36 andrecorded in the recorder 43. The pressure changes (a decrease intemperature resulting from the hydrogen absorption by thehydrogen-absorbing alloy) due to the hydrogen absorption at 25° C. bythe Mg₂Ni hydrogen-absorbing alloy before and after the surfacemodification are shown in FIG. 9.

As apparent from FIG. 9, in the case of the Mg₂Ni hydrogen-absorbingalloy which was not surface-modified, any substantial pressure changewas not recognized even if a predetermined hydrogen pressure was appliedthereto, thus maintaining a constant pressure. Meanwhile, in the case ofthe Mg₂Ni hydrogen-absorbing alloy which was surface-modified, theinternal pressure thereof was abruptly altered by the application of thehydrogen pressure, thus making it possible to confirm a large amount ofhydrogen being absorbed therein. Moreover, it is possible to lower thehydrogen-absorbing temperature as compared with the conventionalhydrogen-absorbing alloy by about 200° C. Namely, in the case of theconventional Mg₂Ni hydrogen-absorbing alloy, thehydrogen-absorption/desorption reaction would not be occurred or wouldbe very slow if occurred unless the temperature is relatively high (200°C. to 300° C.). by contrast, with the employment of thehydrogen-absorbing alloy which has been surface-modified as in Example75, it has been made possible to carry out thehydrogen-absorption/desorption reaction at around room temperature.

EXAMPLES 76-106

The hydrogen-absorbing alloys having compositions as shown in followingTables 13 to 15 were subjected to a surface-modification in the samemanner as in the case of Example 75 to investigate changes in theirhydrogen-absorbing properties before and after the surface modification.In this evaluation, aforementioned hydrogen-absorption/desorptionproperty-evaluating apparatus shown in FIG. 6 was employed. The resultsare shown in the following Tables 13 to 15. In these Tables, the symbolof X indicates the number of hydrogen, i.e., MHx absorbed in thehydrogen-absorbing alloy.

TABLE 13 Hydrogen- absorbing X (Before X (After alloys treatment)treatment) Example 75 Mg₂Ni 0 3.0 Example 76 Mg₂Cu 0 2.0 Example 77Mg₂Co 0 3.5 Example 78 Mg₂Fe 0 4.0 Example 79 LaNi₅ 0.1 6.0 Example 80MmNi₅ 0 3.0 Example 81 CaNi₅ 0 4.0 Example 82 TiFe 0.1 0.5 Example 83TiCo 0 0.5 Example 84 ZrMn₂ 0.1 3.0 Example 85 ZrNi₂ 0.1 3.0

TABLE 14 X (Before X (After Hydrogen-absorbing alloys treatment)treatment) Example 86 Mg₂Ni_(0.8)Co_(0.2) 0 2.3 Example 87Mg₂Ni_(0.9)Co_(0.2) 0 2.9 Example 88 Mg_(2.1)Ni_(1.8)Fe_(0.1) 0 3.0Example 89 Mg₂Ni_(0.7)Mo_(0.2)Rh_(0.2) 0 3.1 Example 90Mg_(1.8)Zr_(0.2)Ni 0 2.5 Example 91 Mg_(1.3)Y_(0.5)Ni 0 2.0 Example 92Mg₂Ir_(0.1)Ni 0 2.4 Example 93 Mg_(1.9)Al_(0.1)Ni_(0.9)Mn_(0.2) 0 3.5Example 94 Mg₂Cu_(0.5)Cd_(0.5) 0 1.6 Example 95 Mg₂Cu_(0.8)Pd_(0.4) 02.0

TABLE 15 X (Before X (After Hydrogen-absorbing alloy treatment)treatment) Example 96 Mg₂Ti_(0.1)Ni 0 2.3 Example 97 Mg₂Nb_(0.1)Ni_(1.2)0 3.1 Example 98 Mg₂Ta_(0.1)Ni_(1.8) 0 2.8 Example 99LaAl_(0.3)Ni_(3.8)Mn_(0.4)Co_(0.5) 0.5 5.0 Example 100NmAl_(0.6)Ni_(3.7)Mn_(0.3)Zr_(0.4) 1.0 5.0 Example 101CaAl_(0.4)Ni_(4.0)Mn_(0.5)Si_(0.1) 0.7 4.5 Example 102TiFe_(0.4)Mn_(0.5) 0.2 2.1 Example 103 TiMn_(1.6)Co_(0.1) 0 2.6 Example104 ZrCo_(1.1)Mn_(1.3) 0.2 3.0 Example 105Zr_(0.6)Ti_(0.4)V_(0.6)Ni_(1.1)Mn_(0.2) 0.1 3.5 Example 106ZrMn_(0.6)V_(0.2)Ni_(1.5)Co_(0.1) 0.2 3.1

As apparent from theses Tables 13 to 15, when a hydrogen-absorbing alloyis surface-modified, the surface thereof is activated so that thehydrogen-absorbing property of the alloy can be improved.

EXAMPLE 107

A Mg₂Ni hydrogen-absorbing alloy was introduced together with stainlesssteel balls into a stainless steel vessel provided with a double cap.Thereafter, the vessel was filled with an argon atmosphere which hasbeen conditioned to contain 1 ppm or less of oxygen and 0.5 ppm or lessof water content. After being sealed with an O-ring, the vessel wassubjected to a ball-milling treatment (a mechanical treatment) for 100hours, the rotational speed thereof being 200 rpm.

This mechanically treated hydrogen-absorbing alloy as well as ahydrogen-absorbing alloy which was not mechanically treated wereinvestigated of their hydrogen-absorbing properties using aforementionedhydrogen-absorption/desorption property-evaluating apparatus shown inFIG. 6. This measurement was conducted by monitoring the pressure changein the reaction vessel when a predetermined volume of hydrogen wasintroduced into the reaction vessel.

Specifically, first of all, each of these hydrogen-absorbing alloys wascharged into the test sample vessel 33. Thereafter, the first valve 37 ₁was closed, and both of the second valve 37 ₂ and the third valve 37 ₃were opened. Under this condition, the vacuum pump 35 was actuated toexhaust the air in each of the pipe 32, the branched pipe 34, thepressure accumulator 38 and the test sample vessel 33. Then, afterclosing the second valve 37 ₂ and the third valve 37 ₃, the first valve37 ₁ was opened to supply hydrogen from the hydrogen gas cylinder 31 toeach of the pipe 32, the branched pipe 34, the pressure accumulator 38and the test sample vessel 33 thereby carrying out a hydrogendisplacement of them. Subsequently, the first valve 37 ₁ was closed andat the same time, the amount of hydrogen introduced was calculated fromthe pressures indicated by the manometer 36. Thereafter, the secondvalve 37 ₂ was opened thereby feeding hydrogen to the test sample vessel33, and the temperature thereof was monitored with the thermocouple 40.At this moment, the thermocouple 40 and the temperature controller 42were controlled so as to keep the temperature inside the test samplevessel 33 constant. At this moment, a change in pressure if any withinthe test sample vessel 33 was detected by means of the manometer 36 andrecorded in the recorder 43.

The hydrogen absorption property at 25° C. of the hydrogen-absorbingalloy particles before and after the mechanical treatment are shown inTable 16 below.

EXAMPLE 108

0.5 mol of Mg₂Ni hydrogen-absorbing alloy and 0.5 mol of Ni powder to befunctioned as a catalyst seed were mixed and the resultant mixture wasintroduced together with stainless steel balls into a stainless steelvessel provided with a double cap. Thereafter, the vessel was filledwith an argon atmosphere which has been conditioned to contain 1 ppm orless of oxygen and 0.5 ppm or less of water content. After being sealedwith an O-ring, the vessel was subjected to a ball-milling treatment (amechanical treatment) for 100 hours, the rotational speed thereof being200 rpm.

This mechanically treated Mg₂Ni hydrogen-absorbing alloy as well as aMg₂Ni hydrogen-absorbing alloy which was not mechanically treated wereinvestigated of their hydrogen-absorbing properties usingafore-mentioned hydrogen-absorption/desorption property-evaluatingapparatus shown in FIG. 6. The hydrogen absorption property at 25° C. ofthe hydrogen-absorbing alloy is shown in Table 16 below.

EXAMPLES 109-150

The hydrogen-absorbing alloys having compositions as shown in followingTables 16 to 19 were subjected to a mechanical treatment in the samemanner as in the case of Example 107 to investigate changes in theirhydrogen-absorbing properties before and after the surface modification.In this evaluation, afore-mentioned hydrogen-absorption/desorptionproperty-evaluating apparatus shown in FIG. 6 was employed. The resultsare shown in the following Tables 16 to 19. In these Tables, the symbolof X indicates the number of hydrogen i.e., MHx absorbed in thehydrogen-absorbing alloy.

TABLE 16 Hydrogen- absorbing X (Before X (After alloys treatment)treatment) Example 107 Mg₂Ni 0 3.0 Example 108 Mg₂Ni 0 3.5 (Ni mixed)Example 109 Mg₂Cu 0 2.5 Example 110 Mg₂Co 0 3.8 Example 111 Mg₂Fe 0 4.4Example 112 LaNi₅ 0.1 6.0 Example 113 MmNi₅ 0 3.5 Example 114 CaNi₅ 04.5 Example 115 TiFe 0.1 1.6 Example 116 TiCo 0 1.8 Example 117 ZrNi₂0.1 3.2 Example 118 ZrNi₂ 0.1 3.6

TABLE 17 X (Before X (After Hydrogen-absorbing alloys treatment)treatment) Example 119 Mg₂Ni_(0.7)Cu_(0.3) 0 2.9 Example 120Mg₂Ni_(0.9)Co_(0.2) 0 3.4 Example 121 Mg₂Ni_(0.8)Fe_(0.1) 0 3.5 Example122 Mg₂Ni_(0.7)Rh_(0.2)Ru_(0.2) 0 3.7 Example 123 Mg_(1.9)Zr_(0.1)Ni 03.0 Example 124 Mg_(1.8)Cr_(0.1)Ni 0 2.6 Example 125 Mg₂Mo_(0.1)Ni 0 2.4Example 126 Mg_(1.9)V_(0.1)Ni_(0.9)Mn_(0.2) 0 3.4 Example 127Mg₂Cu_(0.5)W_(0.5) 0 1.6 Example 128 Mg₂Cu_(0.7)Cd_(0.4) 0 2.0

TABLE 18 X (Before X (After Hydrogen-absorbing alloy treatment)treatment) Example 129 Mg₂Y_(0.1)Ni_(1.1) 0 2.3 Example 130Mg₂Ir_(0.1)Ni_(1.5) 0 3.1 Example 131 Mg₂Pt_(0.1)Ni_(1.9) 0 2.8 Example132 LaAl_(0.3)Ni_(3.5)Mn_(0.4)Co_(0.7) 0.5 6.0 Example 133MmAl_(0.3)Ni_(4.1)Mn_(0.3)Co_(0.3) 1.0 6.0 Example 134CaAl_(0.3)Ni_(4.3)Mn_(0.4) 0.6 5.5 Example 135 TiFe_(0.6)Mn_(0.3) 0.32.1 Example 136 TiMn_(0.6)Co_(0.4) 0 2.5 Example 137 ZrCo_(0.9)Mn_(1.1)0.1 3.5 Example 138 Zr_(0.5)Ti_(0.5)V_(0.7)Ni_(1.3) 0.2 3.9 Example 139ZrMn_(0.5)V_(0.3)Ni_(1.5) 0.1 3.4

TABLE 19 X (Before X (After Hydrogen-absorbing alloy treatment)treatment) Example 140 (Mg_(0.8)Al_(0.2))₃Ni 0 2.0 Example 141(Mg_(0.6)V_(0.4))₈Fe_(0.7)Cu_(0.3) 0 2.6 Example 142(Mg_(0.5)Ba_(0.25)Cr_(0.25))₁₀Co 0 3.1 Example 143(Mg_(0.9)Mn_(0.05)Ti_(0.05))₄Si_(0.6)Zn_(0.4) 0 2.8 Example 144(Mg_(0.7)Mo_(0.3))₅Cu_(0.8)Ni_(0.2) 0 2.5 Example 145(Mg_(0.8)Ca_(0.2))₁₁Ni 0 2.2 Example 146(Mg_(0.5)Sr_(0.5))₆Co_(0.5)Fe_(0.5) 0 3.0 Example 147(Mg_(0.7)Li_(0.3))₇Zn_(0.5)Ni_(0.5) 0 2.1 Example 148(Mg_(0.8)La_(0.1)Y_(0.1))₁₅Cu 0 2.9 Example 149(Mg_(0.8)Na_(0.05)K_(0.15))₉Ni_(0.9)Cu_(0.1) 0 3.2 Example 150(Mg_(0.6)Sr_(0.2)La_(0.2))₁₃Ni_(0.8)Cu_(0.2)Co_(0.2) 0 2.9

As apparent from these Tables 16 to 19, when a hydrogen-absorbing alloyis mechanically treated, the surface thereof is activated so that thehydrogen-absorbing property of the alloy can be improved.

EXAMPLE 151

The hydrogen-absorbing alloy powders obtained from Example 108 andelectrolytic copper powders were mixed in the ratio of 1:1, and 1 g ofthe resultant mixture was subjected to compression for 3 minutes byapplying a pressure of 20 tons in a table-molding device (innerdiameter: 10 mm) thereby obtaining a pellet. This pellet was theninterposed between Ni wire nettings, and the peripheral portion thereofwas spot-welded and pressed. Subsequently, to this pressed body wasconnected Ni lead wires by means of spot-welding thereby preparing ahydrogen-absorbing alloy electrode (a negative electrode).

COMPARATIVE EXAMPLE 20

A hydrogen-absorbing alloy electrode (a negative electrode) was preparedin the same manner as explained in Example 151 except that a Mg₂Nihydrogen-absorbing alloy which was not subjected to the mechanicaltreatment was used as a raw material.

The negative electrodes of Example 151 and Comparative Example 20 weredipped into a 8N aqueous solution of potassium hydroxide together withcounter electrodes (sintered nickel electrodes), and then, acharge/discharge cycle test was performed at a temperature of 25° C. Inthis charge/discharge cycle test, the charging was conducted using acurrent of 100 mA per 1 g of hydrogen-absorbing alloy for 10 hours, andafter a cessation of 10 minutes, the discharge was conducted using acurrent of 100 mA per 1 g of hydrogen-absorbing alloy until the voltageagainst the mercury oxide electrode was lowered down to −0.5 V. Thischarge/discharge cycle was repeated. The result of this charge/dischargecycle is shown in FIG. 10. In this FIG. 10, the symbol A indicates acharge/discharge property line of Comparative Example 20 employing anegative electrode which was not mechanically treated, while the symbolB indicates a charge/discharge property line of Example 151 employing anegative electrode which was mechanically treated.

As apparent from FIG. 10, in the case of Comparative Example 20(charge/discharge property line A), it was impossible to carry out thecharge/discharge at normal temperature, indicating no dischargecapacity. By contrast, in the case of Example 151 (charge/dischargeproperty line B), a discharge capacity of 750 mAh/g was indicated fromthe first cycle, thus making it clear that it is possible, through thismechanical treatment, to prominently increase the discharge capacity.Therefore, it can be said that the mechanical treatment is an effectiveway to prominently improve the discharge property of a battery providedwith a negative electrode containing a hydrogen-absorbing alloy.

EXAMPLES 152-155, AND COMPARATIVE EXAMPLE 20

A Mg₂Ni hydrogen-absorbing alloy was introduced together with stainlesssteel balls into a stainless steel vessel provided with a double cap.Thereafter, the vessel was filled with an argon atmosphere which hasbeen conditioned to contain 1 ppm or less of oxygen and 0.5 ppm or lessof water content. After being sealed with an O-ring, the vessel wassubjected to a ball-milling treatment (a mechanical treatment) for 2hours, 50 hours, 200 hours and 800 hours respectively with therotational speed thereof being controlled to 200 rpm, thereby obtainingfour kinds of surface-modified hydrogen-absorbing alloy powders.

This mechanically treated hydrogen-absorbing alloy as well as ahydrogen-absorbing alloy which was not mechanically treated wereemployed to prepare a hydrogen-absorbing alloy electrode (a negativeelectrode) in the same manner as explained in Example 151. Thesenegative electrodes were dipped into a 8N aqueous solution of potassiumhydroxide together with counter electrodes (sintered nickel electrodes),and then, a charge/discharge cycle test was performed at a temperatureof 25° C. In this charge/discharge cycle test, the charging wasconducted in the same manner as explained in Example 151, and themaximum discharge capacity thereof were measured. The result of thischarge/discharge cycle is shown in Table 20. In this Table 20, anaverage particle diameter of each of these hydrogen-absorbing alloypowders and a hydrogen-absorbing alloy powder used Comparative Example20 is also indicated.

TABLE 20 Average particle Discharge Treatment diameter capacity time (h)(μm) (mAh/g) Comparative 0 80 0 Example 20 Example 152 2 20 115 Example153 50 6 523 Example 154 200 2 617 Example 155 800 1 658

As apparent from Table 20, with the increase of the mechanical treatmenttime, the average particle diameter of the hydrogen-absorbing alloy wasproportionally minimized, thereby making it possible to increase thedischarge capacity.

EXAMPLES 156-162 AND COMPARATIVE EXAMPLE 20

A Mg₂Ni hydrogen-absorbing alloy was introduced together with stainlesssteel balls into a stainless steel vessel provided with a double cap.Thereafter, the vessel was filled with an argon atmosphere which hasbeen conditioned to contain 1 ppm or less of oxygen and 0.5 ppm or lessof water content. After being sealed with an O-ring, the vessel wassubjected to a ball-milling treatment (a mechanical treatment) for 3hours, 40 hours, 300 hours, 650 hours, 800 hours, 900 hours and 1000hours respectively with the rotational speed thereof being controlled to200 rpm, thereby obtaining four kinds of surface-modifiedhydrogen-absorbing alloy powders.

This mechanically treated hydrogen-absorbing alloy as well as ahydrogen-absorbing alloy which was not mechanically treated wereemployed to prepare a hydrogen-absorbing alloy electrode (a negativeelectrode) in the same manner as explained in Example 151. Thesenegative electrodes were dipped into a 8N aqueous solution of potassiumhydroxide together with counter electrodes (sintered nickel electrodes),and then, a charge/discharge cycle test was performed at a temperatureof 25° C. In this charge/discharge cycle test, the charging wasconducted in the same manner as explained in Example 151, and themaximum discharge capacity thereof were measured. The result of thischarge/discharge cycle is shown in Table 21. In this Table 21, a valueof Δ(2θ₂) (a half-width of at least one peak out of peaks of threestrongest lines to be obtained by an X-ray diffraction using Cukα-ray asa radiation source) of the hydrogen-absorbing alloy used Examples 156 to162 and Comparative Example 20 is also indicated.

TABLE 21 Discharge Treatment Δ (2 θ₂) capacity time (h) (°) (mAh/g)Comparative 0 0.1 0 Example 20 Example 156 3 0.5 142 Example 157 40 1.2503 Example 158 300 3.4 631 Example 159 650 5.1 649 Example 160 800 9.5653 Example 161 900 25.1 630 Example 162 1000 41.3 535

As apparent from Table 21, with the increase of the mechanical treatmenttime, the crystal grain size of the hydrogen-absorbing alloy particleswas proportionally minimized and at the same time, the value of Δ(2θ₂)was increased, thereby making it possible to increase the dischargecapacity.

EXAMPLES 163-167, AND COMPARATIVE EXAMPLE 20

A Mg₂Ni hydrogen-absorbing alloy was introduced together with stainlesssteel balls into a stainless steel vessel provided with a double cap.Thereafter, the vessel was filled with an argon atmosphere which hasbeen conditioned to contain 1 ppm or less of oxygen and 0.5 ppm or lessof water content. After being sealed with an O-ring, the vessel wassubjected to a ball-milling treatment 8 a mechanical treatment) for 0.5hours, 2.5 hours, 15 hours, 250 hours and 700 hours respectively withthe rotational speed thereof being controlled to 200 rpm, therebyobtaining five kinds of surface-modified hydrogen-absorbing alloypowders.

This mechanically treated hydrogen-absorbing alloy as well as ahydrogen-absorbing alloy which was not mechanically treated wereemployed to prepare a hydrogen-absorbing alloy electrode (a negativeelectrode) in the same manner as explained in Example 151. Thesenegative electrodes were dipped into a 8N aqueous solution of potassiumhydroxide together with counter electrodes (sintered nickel electrodes),and then, a charge/discharge cycle test was performed at a temperatureof 25° C. In this charge/discharge cycle test, the charging wasconducted in the same manner as explained in Example 151, and themaximum discharge capacity thereof were measured. The result of thischarge/discharge cycle is shown in Table 22. In this Table 22, the sizeof crystal grain of each of these hydrogen-absorbing alloy powders usedExamples 163 to 167 and Comparative Example 20 is also indicated.

TABLE 22 Size of Discharge Treatment crystallite capacity time (h) (nm)(mAh/g) Comparative 0 111.5 0 Example 20 Example 163 0.5 56.2 13 Example164 2.5 37.5 124 Example 165 15 23.2 323 Example 166 250 5.1 626 Example167 700 2.2 651

As apparent from Table 22, with the increase of the mechanical treatmenttime, the size of crystal grain of the hydrogen-absorbing alloy wasproportionally minimized, thereby making it possible to increase thedischarge capacity.

EXAMPLES 168-172, AND COMPARATIVE EXAMPLES 21 AND 22

A Mg₂Ni hydrogen-absorbing alloy was introduced together with stainlesssteel balls into a stainless steel vessel provided with a double cap.Thereafter, the vessel was filled with seven kinds of atmosphere, i.e.,vacuum, an inert gas (argon, nitrogen or helium), hydrogen, oxygen andair. After being sealed with an O-ring, the vessel was subjected to aball-milling treatment (a mechanical treatment) for 100 hours with therotational speed thereof being controlled to 200 rpm, thereby obtainingseven kinds of surface-modified hydrogen-absorbing alloy powders.

These mechanically treated hydrogen-absorbing alloys were employed toprepare a hydrogen-absorbing alloy electrode (a negative electrode) inthe same manner as explained in Example 151. These negative electrodeswere dipped into a 8N aqueous solution of potassium hydroxide togetherwith counter electrodes (sintered nickel electrodes), and then, acharge/discharge cycle test was performed at a temperature of 25° C. inthe same manner as explained in Example 151, and the maximum dischargecapacity thereof were measured. The result of this charge/dischargecycle is shown in Table 23.

TABLE 23 Discharge capacity Treatment atmosphere (mAh/g) Example 168Argon (99.999%) 605 Example 169 Vacuum 402 Example 170 Nitrogen(99.999%) 513 Example 171 Helium (99.999%) 526 Example 172 Hydrogen(99.99999%) 650 Comparative Oxygen (99.999%) 0 Example 21 ComparativeAir 0 Example 22

As apparent from Examples 168 to 172 of Table 23, it is desirable toperform the mechanical treatment in vacuum atmosphere or in anatmosphere of an inert gas or hydrogen. It is possible to increase thedischarge capacity if the mechanical treatment is performed in

TABLE 24 Alloy having Treat- Dis- hydrogen- ment charge absorbing Addi-time capacity properties tive (h) (mAh/g) Compara- Mg₂Ni — — 0 tiveExample 20 Example Mg₂Ni Ni 100 751 173 Example Mg₂Ni — 1 78 174 ExampleMg₂Ni — 25 452 175 Example Mg₂Ni — 100 605 176 Example Mg₂Ni Ni 500 825177 Example Mg₂Ni Co 100 752 178 Example Mg₂Ni Fe 100 703 179 ExampleMg₂Ni WCO₃ 100 642 180 Example Mg₂Ni IrO₂ 100 750 181 Example Mg₂Cu Cu100 502 182

such a preferable atmosphere.

EXAMPLES 173-219 AND COMPARATIVE EXAMPLE 20

The hydrogen-absorbing alloys having compositions as shown in followingTables 24 to 27 were subjected to a mechanical treatment under variousconditions thereby effecting a surface treatment.

These mechanically treated hydrogen-absorbing alloys were employed toprepare negative electrodes comprising these hydrogen-absorbing alloysin the same manner as explained in Example 151. Then, these negativeelectrodes were dipped into a 8N aqueous solution of potassium hydroxidetogether with counter electrodes (sintered nickel electrodes), and then,a charge/discharge cycle test was performed at a temperature of 25° C.in the same manner as explained in Example 151, and the maximumdischarge capacity thereof were measured. The results are shown in Table24 to 27. In these Tables 24 to 27, the maximum discharge capacity ofComparative Example 20 is also indicated.

TABLE 25 Alloy having Treat- Dis- hydrogen- ment charge absorbing Addi-time capacity properties tive (h) (mAh/g) Example Mg₂CO CO 100 712 183Example Mg₂Fe Fe 100 745 184 Example LaNi₅ — — 274 185 Example LaNi₅ Pt100 321 186 Example LaNi₅ MoCo₃ 100 325 187 Example LaNi₅ CoO 100 290188 Example MmNi₅ Rh 100 123 189 Example CaNi₅ MoNi₃ 100 150 190 ExampleTiFe Pd 100 154 191 Example TiCo FeO 100 111 192 Example ZrMn₂ Wni₃ 100148 193 Example ZrNi₂ Au 100 85 194

TABLE 26 Alloys Discharge having hydrogen- Treatment capacity absorbingproperties Additive time (h) (mAh/g) Example 195 Mg₂Ni_(0.8)Co_(0.2) Ag100 650 Example 196 Mg₂Ni_(0.6)Co_(0.5) Ir 100 730 Example 197Mg₂Ni_(0.7)Fe_(0.2) V 50 360 Example 198 Mg₂Al_(0.2)Ni_(0.8)Mn_(0.2) CoO100 850 Example 199 Mg_(1.9)B_(0.1)Ni NiO 100 605 Example 200Mg_(1.8)C_(0.1)Ni Pd 500 625 Example 201 Mg₂Au_(0.1)Ni Cr 200 652Example 202 Mg_(1.8)Al_(0.2)Ni_(0.8)Cr_(0.2) Ni 100 800 Example 203Mg₂Cu_(0.8)Co_(0.2) Mn 100 503 Example 204 Mg₂Cu_(0.7)Sn_(0.5) Co₃O₄ 100524 Example 205 Mg₂Au_(0.1)Ni_(1.3) Ru 100 605 Example 206Mg₂Ni_(1.6)Ag_(0.1) Mo 100 553 Example 207 Mg₂Al_(0.1)Ni_(1.9) RhO₂ 100502

TABLE 27 Treat- Discharge Alloy having hydrogen- ment capacity absorbingproperties Additive time (h) (mAh/g) Example 208Mg₂Fe_(0.5)Ni_(0.6)Zn_(0.1) W 100 645 Example 209LaAl_(0.3)Ni_(3.7)Mn_(0.5)Co_(0.5) VCO₃ 100 285 Example 210LaAl_(0.4)Ni_(4.4)Zr_(0.2) Ru 100 231 Example 211LaAl_(0.3)Ni_(3.7)Mn_(0.5)Co_(0.5) VNi₃ 100 265 Example 212MmAl_(1.0)Ni_(3.5)Si_(0.5) Nb 100 284 Example 213MmNi_(3.6)Mn_(0.4)Ti_(0.3)Co_(0.7) WPt₃ 100 205 Example 214MmAl_(0.2)Ni_(3.8)Mn_(0.5)Cu_(0.5) Ta 100 250 Example 215TiFe_(0.8)Mn_(0.1) Co₂O₃ 100 211 Example 216 TiCo_(0.6)Mn_(0.5) V 100260 Example 217 Zr_(0.6)Ti_(0.4)V_(0.6)Ni_(1.3) Au 100 390 Example 218ZrCo_(1.0)Mn_(1.3) RuO₂ 100 350 Example 219 ZrV_(0.3)Ni_(1.4)Mn_(0.6) Ta100 380

As apparent from these Tables 24 to 27, when a hydrogen-absorbing alloyis mechanically treated, the discharge capacity thereof can be increasedso that the charge/discharge property of the alloy can be prominentlyimproved.

EXAMPLE 220

Mg₂Ni alloy obtained by way of high-frequency melting method was mixedwith 20% by volume (based on Mg₂Ni alloy) of Ni powder to be functionedas a catalyst seed (an additive), and the resultant mixture wasintroduced together with stainless steel balls into a stainless steelvessel provided with a double cap. Thereafter, the vessel was filledwith an argon atmosphere which has been conditioned to contain 1 ppm orless of oxygen and 0.5 ppm or less of water content. After being sealedwith an O-ring, the vessel was subjected to a ball-milling treatment (amechanical treatment) for 100 hours, the rotational speed thereof beingcontrolled to 200 rpm.

Then, a measurement was performed regarding the hydrogen-absorbing rateat 25° C. of the resultant alloy by making use of the aforementionedhydrogen-absorption/desorption property-evaluating apparatus shown inFIG. 6. The hydrogen absorption rate V of the alloy is indicated by thenumber of hydrogen MHV which was absorbed in the alloy for a time periodof 10 hours starting from the introduction of predetermined quantity ofhydrogen into the reaction vessel. The results are shown in Table 28below.

EXAMPLES 221-241

As shown in Table 28, the alloys having hydrogen-absorbing propertieswere mixed respectively with percent by volume (based on thehydrogen-absorbing alloys) of an additive to be functioned as a catalystseed, and the resultant mixtures were subjected to a mechanicaltreatment under the same conditions as in the case of Example 220.

Thereafter, the hydrogen-absorbing rate at 25° C. of eachhydrogen-absorbing alloy was performed in the same manner as explainedin Example 220 by making use of the aforementionedhydrogen-absorption/desorption property-evaluating apparatus shown inFIG. 6. The results on these mechanically treated hydrogen-absorbingalloys are shown in the following Table 28 together with the result onan Mg₂Ni alloy which was not mechanically treated (Comparative Example20).

TABLE 28 Alloy Amount V V having hydrogen- of (Before (After absorbingadditive treat- treat- properties Additive (vol %) ment) ment)Comparative Mg₂Ni — — 0 — Example 20 Example 220 Mg₂Ni Ni 20 0 3.3Example 221 Mg₂Ni_(0.5)Cu_(0.5) Pd 51 0 2.7 Example 222Mg₂Ni_(0.75)Co_(0.25) MoCo₃ 32 0 3.9 Example 223 Mg₂Ni_(0.75)Fe_(0.25)RuO₃ 61 0 3.2 Example 224 Ti₂Ni MmNi₅ 5 0 1.9 Example 225 LaNi₅ V 37 0.16.0 Example 226 MmNi₅ MgZn₂ 21 0 3.6 Example 227 CaNi₅ Hf 9 0 4.8Example 228 MgNi₂ La₃Ni 42 0 3.4 Example 229 VNi₂ ZrFe₂ 13 0 3.7 Example230 TiNi Ir 46 0 1.9 Example 231 LaNi V₄Ti 84 0.1 2.6 Example 232 Vni Ni29 0.1 2.3 Example 233 LaNi₃ Ca₂Fe 34 0.1 4.2 Example 234 Vni₃ Mg₂Ni 550 4.1 Example 235 La₂Ni₇ Pt 42 0.1 7.6 Example 236 Zr₂Ni₇ Mg₂Cu 63 0 6.9Example 237 La₂Ni₃ ZrNi₂ 2 0.2 3.6 Example 238 Ca₂Ni₃ Mo 27 0.1 3.1Example 239 La₇Ni₃ TiNi 13 0.1 4.2 Example 240 La₃Ni Co₃O₄ 73 0.2 8.1Example 241 V₃Ni Co 58 0 7.6

As apparent from Table 28, the hydrogen-absorbing alloys of Examples 220to 241 which were obtained by mechanically treating a mixture comprisinga hydrogen-absorptive alloy containing not less than 5% by volume of Niand mixed with an additive such as a metallic additive exhibited aprominently high hydrogen-absorbing rate and improved hydrogen-absorbingproperties as compared with the hydrogen-absorbing alloy of ComparativeExample 19 which was formed of an Mg₂Ni alloy and not subjected to theaforementioned mechanical treatment.

EXAMPLE 242

The powdered hydrogen-absorbing alloy obtained in Example 220 was mixedwith electrolytic copper powder in the weight ratio of 1:1, and 1 g ofthe resultant mixture was subjected to compression for 3 minutes byapplying a pressure of 20,000 kg in a tablet-molding device (innerdiameter: 10 mm) thereby obtaining a pellet. This pellet was thensandwiched between Ni wire nettings, and the peripheral portion thereofwas spot-welded and pressed. Subsequently, to this pressed body wasconnected a Ni lead wire by means of spot-welding thereby preparing ahydrogen-absorbing alloy electrode (a negative electrode).

The hydrogen electrode thus obtained was dipped, together with asintered nickel electrode constituting a counter electrode, into a 8Naqueous solution of potassium hydroxide, and then, a charge/dischargecycle test was performed at a temperature of 25° C. In thischarge/discharge cycle test, each cycle was consisted of the steps, i.e.the charging was conducted under the condition of 100 mA per 1 g ofhydrogen-absorbing alloy for 10 hours, and, after a ten minutecessation, the discharge was then conducted under the condition of 20 mAper 1 g of hydrogen-absorbing alloy until the voltage against themercury oxide electrode was lowered down to −0.5 V.

The cycling characteristic in discharge capacity of the negativeelectrode of the Comparative Example 20 (containing Mg₂Ni which was notsubjected to the aforementioned mechanical treatment) and the cyclingcharacteristic in discharge capacity of the negative electrode ofExample 242 are indicated in FIG. 11 by the curves a and b,respectively. As apparent from FIG. 11, the negative electrode (thecurve a) containing Mg₂Ni which was not subjected to the aforementionedmechanical treatment failed to perform any charging/discharging at thenormal temperature, thus indicating little discharge capacity. On theother hand, the negative electrode (the curve b) containing thehydrogen-absorbing alloy of Example 220 indicated a discharge capacityof 832 mAh/g from the first cycle, thus indicating a prominent increasein discharge capacity that had been brought about by the mechanicaltreatment.

EXAMPLES 243-253

Twelve kinds of hydrogen-absorbing alloy were prepared by mixing nickelat a volume ratios shown in the following Table 29 with Mg₂Ni alloy, andthe resultant mixtures were treated in the same manner as in Example220.

Thereafter, the hydrogen-absorbing rate of each hydrogen-absorbing alloythus obtained was measured in the same manner as in Example 220.

Furthermore, negative electrodes were prepared in the same manner asexplained in Example 242 by making use of each hydrogen-absorbing alloy,and the negative electrodes thus obtained were respectively dipped,together with a sintered nickel electrode constituting a counterelectrode, into a 8N aqueous solution of potassium hydroxide, and then,a charge/discharge cycle test was performed at a temperature of 25° C.to measure the maximum discharge capacity.

The results obtained are shown in Table 29 together with the result ofComparative Example 20.

FIG. 12 indicates XRD patterns (A to G) which were obtained as thecontent of nickel was altered. In FIG. 12, A indicates the pattern ofthe sample Mg₂Ni+5 vol. % Ni; B, the pattern of the sample Mg₂Ni+10 vol.% Ni; C, the pattern of the sample Mg₂Ni+15 vol. % Ni; D, the pattern ofthe sample Mg₂Ni+18 vol. % Ni; E, the pattern of the sample Mg₂Ni+22vol. % Ni; F, the pattern of the sample Mg₂Ni+25 vol. % Ni; and G, thepattern of the sample Mg₂Ni+33 vol. % Ni. As apparent from FIG. 12, asthe content of nickel was increased, the peaks in the vicinity of 20°and in the vicinity of 40° in the XRD patterns of Mg₂Ni becomeproportionally broader, thus prominently increasing the apparenthalf-widths α(2θ_(20°) and Δ(2θ_(40°)).

TABLE 29 Amount of Hydrogen Discharge capacity nickel (vol %) absorptionrate V (mAh/g) Comparative 0 0 0 Example 20 Example 243 5.1 2.1 525Example 244 9.7 2.2 544 Example 245 13.9 2.5 645 Example 246 17.7 3.0751 Example 247 21.2 3.1 790 Example 248 24.4 3.2 805 Example 249 30.03.3 832 Example 250 34.5 3.4 850 Example 251 39.2 3.6 900 Example 25249.8 3.7 920 Example 253 65.9 2.8 700

As apparent from Table 29, the hydrogen-absorbing rate as well as thedischarge capacity could be increased by increasing the content ofnickel up to 50 vol. %. However, when the content of nickel exceededover 50 vol. %, the hydrogen-absorbing rate was greatly decreased on thecontrary.

EXAMPLES 254-275

22 kinds of hydrogen-absorbing alloy were prepared by mixing variouskinds of additive at a volume ratios with various kinds of alloy havinghydrogen-absorbing properties as shown in the following Table 30, andthe resultant mixtures were mechanically treated in the same manner asin Example 220 to prepare 22 kinds of hydrogen-absorbing alloys.

Thereafter, negative electrodes were prepared in the same manner asexplained in Example 242 by making use of each hydrogen-absorbing alloy,and the negative electrodes thus obtained were respectively dipped,together with a sintered nickel electrode constituting a counterelectrode, into a 8N aqueous solution of potassium hydroxide, and then,a charge/discharge cycle test was performed at a temperature of 25° C.to measure the maximum discharge capacity.

The results obtained are shown in Table 30 together with the result ofComparative Example 20.

TABLE 30 Alloy having hydrogen- Amount of Discharge absorbing additivecapacity properties Additive (vol %) (mAh/g) Comparative Mg₂Ni — — 0Example 20 Example 254 Mg₂Ni Ni 65 820 Example 255 Mg₂Ni_(0.5)Cu_(0.5)WNi₃ 31 728 Example 256 Mg₂Ni_(0.75)Co_(0.25) NiO 27 826 Example 257Mg₂Ni_(0.75)Co_(0.25) LaNi₅ 48 850 Example 258 Ti₂Ni Au 3 302 Example259 LaNi₅ V₂O₅ 63 350 Example 260 MmNi₅ ZrFe₂ 22 285 Example 261 CaNi₅Os 38 350 Example 262 MgNi₂ TiNi 52 442 Example 263 VNi₂ Wco₃ 8 419Example 264 TiNi Zr₂Ni₇ 26 374 Example 265 LaNi Ti 64 355 Example 266VNi Ni 14 480 Example 267 LaNi₃ RuO₂ 17 450 Example 268 VNi₃ Co 56 401Example 269 La₂Ni₇ Ta 35 360 Example 270 Zr₂Ni₇ Na₂Ni 2 364 Example 271La₂Ni₃ ZrNi₂ 42 222 Example 272 Ca₂Ni₃ LaNi 29 211 Example 273 La₇Ni₃V₃Ni 51 503 Example 274 La₃Ni Co₂O₃ 38 450 Example 275 V₃Ni Rf 7 700

As apparent from Table 30, it was found possible by the mechanicaltreatment of the hydrogen-absorbing alloys to increase the dischargecapacity and to greatly improve the charge/discharge characteristics ofthe alloys.

EXAMPLES 276-279 AND COMPARATIVE EXAMPLE 20

Mg₂Ni alloy obtained by way of high-frequency melting method and LaNi₅alloy obtained by way of high-frequency melting method were mixedtogether at a volume ratio of 80:20, and the resultant mixtures weresubjected to the same mechanical treatment as in the case of Example 220for various time periods, thereby preparing four different kinds ofhydrogen-absorbing alloy.

Then, the value of Δ(2θ₂) (a half-width of a peak in the vicinity of 40°in an X-ray diffraction using CuK_(α)-ray as a radiation source) of eachhydrogen-absorbing alloy and the hydrogen-absorbing rate of eachhydrogen-absorbing alloy were measured respectively.

Thereafter, negative electrodes were prepared in the same manner asexplained in Example 242 by making use of each hydrogen-absorbing alloy,and the negative electrodes thus obtained were respectively dipped,together with a sintered nickel electrode constituting a counterelectrode, into a 8N aqueous solution of potassium hydroxide, and then,a charge/discharge cycle test was performed at a temperature of 25° C.to measure the maximum discharge capacity.

The results obtained are shown in Table 31 together with the result ofComparative Example 20 where a Mg₂Ni alloy was not subjected to theaforementioned mechanical treatment.

TABLE 31 Treatment Discharge time Hydrogen capacity (h) Δ(2 θ₂)(°)absorption rate V (mAh/g) Comparative 0 0.07 0.6 150 Example 20 Example276 5 0.60 1.9 463 Example 277 50 1.50 2.4 621 Example 278 400 3.70 3.4860 Example 279 700 6.10 3.5 871

As apparent from Table 31, it was found that when this mechanicaltreatment was performed for a longer period of time, the particlediameter of the crystallite became smaller, and at the same time, anon-uniform distortion was generated within the crystal, therebyincreasing the value of Δ(2θ₂) and greatly increasing thehydrogen-absorbing rate and discharge capacity of the alloys.

EXAMPLES 280-284, AND COMPARATIVE EXAMPLES 23 AND 24

70 vol. % of a Mg₂Ni hydrogen-absorbing alloy and 30 vol. % of Co powderwere introduced together with stainless steel balls into a stainlesssteel vessel provided with a double cap. Thereafter, the vessel wasfilled with seven kinds of atmosphere, i.e., vacuum, an inert gas(argon, nitrogen or helium), hydrogen, oxygen and air. The mechanicaltreatment was performed in the same manner as in the case of Example 220to prepare seven kinds of surface-modified hydrogen-absorbing alloypowders.

Then, the hydrogen-absorbing rate of each modified hydrogen-absorbingalloy was measured by making use of these modified hydrogen-absorbingalloy powders.

Thereafter, negative electrodes were prepared in the same manner asexplained in Example 242 by making use of each of these modifiedhydrogen-absorbing alloy, and the negative electrodes thus obtained wererespectively dipped, together with a sintered nickel electrodeconstituting a counter electrode, into a 8N aqueous solution ofpotassium hydroxide, and then, a charge/discharge cycle test wasperformed at a temperature of 25° C. to measure the maximum dischargecapacity.

The results obtained are shown in Table 32.

TABLE 32 Discharge Hydrogen capacity Treatment atmosphere absorptionrate V (mAh/g) Example 280 Argon (99.999%) 2.4 610 Example 281 Vacuum2.0 415 Example 282 Nitrogen (99.999%) 2.2 550 Example 283 Helium(99.999%) 2.1 531 Example 284 Hydrogen (99.99999%) 2.7 672 ComparativeOxygen (99.999%) 0 0 Example 23 Comparative Air 0 0 Example 24

As apparent from Table 32, it was found that the employment of vacuum,an inert gas or hydrogen gas was,preferable as an atmosphere for themechanical treatment, i.e. hydrogen-absorbing alloys which weremechanically treated in these atmospheres exhibited a greatly increasedhydrogen-absorbing rate, and the negative electrode containing any ofthese hydrogen-absorbing alloys exhibited an increased dischargecapacity.

EXAMPLES 285-306

Powdered additives having a predetermined particle diameter were addedto the alloys having hydrogen-absorbing properties as shown in thefollowing Table 33, and the resultant mixtures were subjected to thesame mechanical treatment as in the case of Example 220 for various timeperiods, thereby preparing 22 different kinds of hydrogen-absorbingalloy.

Then, the hydrogen-absorbing rate of each hydrogen-absorbing alloy wasmeasured in the same manner as in Example 220.

Thereafter, negative electrodes were prepared in the same manner asexplained in Example 242 by making use of each hydrogen-absorbing alloy,and the negative electrodes thus obtained were respectively dipped,together with a sintered nickel electrode constituting a counterelectrode, into a 8N aqueous solution of potassium hydroxide, and then,a charge/discharge cycle test was performed at a temperature of 25° C.to measure the maximum discharge capacity.

The results obtained are shown in Table 33 together with the dispersionvolume of the aforementioned powdered additives and with the result ofComparative Example 20.

TABLE 33 Alloy having hydrogen- Particle Dispersion Hydrogen Dischargeabsorbing diameter ratio absorption capacity properties Additive (μm)(vol %) rate V (mAh/g) Comparative Mg₂Ni — — 0 0 0 Example 20 Example285 Mg₂Ni Ni 0.001 30 3.0 743 Example 286 Mg₂Ni_(0.6)Cu_(0.4) Zr 0.51317 2.8 700 Example 287 Mg₂Ni_(0.7)Cu_(0.3) MoNi₃ 8.15 0.4 3.3 824Example 288 Mg₂Ni_(0.85)Fe_(0.15) Co2O₃ 0.692 16 3.4 856 Example 289Ti₂Ni Ag 3.68 0.7 3.0 290 Example 290 LaNi₅ ZrNi₂ 5.1 30 5.0 381 Example291 MmNi₅ W 35.7 4.3 5.5 256 Example 292 CaNi₅ ZrFe₃ 0.325 17 5.8 348Example 293 MgNi₂ V 0.013 32 3.1 431 Example 294 VNi₂ Mg₂Ni 12.0 0.5 3.8409 Example 295 TiNi Tc 1.35 2.5 2.0 368 Example 296 LaNi Mg₂Cu 2.31 262.7 351 Example 297 VNi LaNi₅ 0.052 1.8 3.0 360 Example 298 LaNi₃ Ti₂Ni0.016 45 4.8 425 Example 299 VNi₃ TiFe 0.894 3.6 4.9 431 Example 300La₂Ni₇ Zr₂Fe 0.953 23 10.1 367 Example 301 Zr₂Ni₇ IrO₂ 22.3 0.02 9.5 356Example 302 La₂Ni₃ MmNi₅ 0.413 8.9 3.6 218 Example 303 Ca₂Ni₃ Rh 40.5 483.8 209 Example 304 La₇Ni₃ WCo₃ 35.36 0.06 4.0 482 Example 305 La₃Ni Ru0.156 31 8.5 503 Example 306 V₃Ni Cr 8.91 15 9.0 690

As apparent from Table 33, it was found that, although thehydrogen-absorbing rate V of Mg₂Ni per se was about 0 to 0.5 at thenormal temperature, when a powdered additive such as Ni was added to analloy having hydrogen-absorbing properties such as thehydrogen-absorbing alloys of Examples 285 to 306, the hydrogen-absorbingproperties at the normal temperature of the alloy could be improved, andat the same time, the charge/discharge characteristics of the negativeelectrode containing this hydrogen-absorbing alloy could be greatlyimproved.

EXAMPLES 307-326

Powdered additives having a predetermined particle diameter were addedto the alloys having hydrogen-absorbing properties and represented bythe aforementioned general formulas (V) and (VI) as shown in thefollowing Table 34, and the resultant mixtures were subjected to thesame mechanical treatment as in the case of Example 220 for various timeperiods, thereby preparing 20 different kinds of hydrogen-absorbingalloy.

Then, the hydrogen-absorbing rate of each hydrogen-absorbing alloy wasmeasured in the same manner as in Example 220.

Thereafter, negative electrodes were prepared in the same manner asexplained in Example 242 by making use of each hydrogen-absorbing alloy,and the negative electrodes thus obtained were respectively dipped,together with a sintered nickel electrode constituting a counterelectrode, into a 8N aqueous solution of potassium hydroxide, and then,a charge/discharge cycle test was performed at a temperature of 25° C.to measure the maximum discharge capacity.

The results obtained are shown in Table 34 together with the dispersionvolume of the aforementioned powdered additives.

TABLE 34 Particle Dispersion Hydrogen Discharge Alloy having hydrogen-diameter ratio absorption capacity absorbing properties Additive (μm)(vol %) rate V (mAh/g) Example 307 (Mg_(0.8)Mn_(0.2))₄Ni Co 5 3.2 2.8456 Example 308 (Mg_(0.6)Cr_(0.4))₁₂Co_(0.3)Cu_(0.7) CaNi₅ 20 15.3 3.0391 Example 309 (Mg_(0.8)Al_(0.1)B_(0.1))₄Fe TiFe 35 8.6 1.6 530 Example310 (Mg_(0.9)Mo_(0.1))₁₁Si Ni 0.1 25.2 3.1 700 Example 311(Mg_(0.7)Ru_(0.3))₁₃Sn_(0.5)Zn_(0.5) V 3 40.1 2.6 621 Example 312(Mg_(0.8)Pd_(0.1)W_(0.1))₁₀Ni Ti₂Ni 18 55.3 2.2 313 Example 313(Mg_(0.7)Zr_(0.3))₅Ni_(0.9)Cu_(0.1) Co₃Mo 50 1.2 2.1 215 Example 314(Mg_(0.9)C_(0.1))₁₅Ni_(0.9)Co_(0.1) Ni 0.9 11.6 2.9 400 Example 315(Mg_(0.7)Ge_(0.3))₉Fe ZrMnNi 23 31.2 3.1 390 Example 316(Mg_(0.8)P_(0.1)Ti_(0.1))₃Cu Pt 11 0.6 2.6 280 Example 317(Mg_(0.9)K_(0.1))₃Ni NiP 0.5 19.3 2.2 431 Example 318(Mg_(0.6)Ca_(0.4))₁₄Co_(0.5)Ni_(0.5) Mo 1.3 60.2 1.3 320 Example 319(Mg_(0.8)Ca_(0.1)Sr_(0.1))₁₂Zn Pd 5 45.3 2.9 800 Example 320(Mg_(0.5)Ba_(0.5))₁₀Cu_(0.7)Ni_(0.3) V₄Ti 13 1.2 3.0 293 Example 321(Mg_(0.8)Na_(0.2))₁₆Fe NiB 8 16.3 2.2 411 Example 322(Mg_(0.7)La_(0.2)Li_(0.1))₅Si LaNi₄Al 15 21.6 2.6 393 Example 323(Mg_(0.8)Y_(0.1)Ca_(0.1))₂₀Ni_(0.8) Ni 0.05 30.3 2.5 516Co_(0.1)Cu_(0.1) Example 324 (Mg_(0.6)Sr_(0.4))₅Cu Au 21 9.6 2.1 290Example 325 (Mg_(0.9)Li_(0.1))₈Ni B 0.8 51.2 1.3 410 Example 326(Mg_(0.6)La_(0.4))₄Co CaAlNi₄ 3 3.2 2.2 391

As apparent from Table 34, it was found that the hydrogen-absorbingalloys according to Examples 307 to 326 wherein a powdered additive suchas Ni was added to the alloys having hydrogen-absorbing properties andrepresented by the aforementioned general formulas (V) and (VI) werecapable of further improving the hydrogen-absorbing properties at thenormal temperature as compared with the hydrogen-absorbing alloys ofExamples 307 to 326.

EXAMPLES 327-332, AND COMPARATIVE EXAMPLES 25-27

To the hydrogen-absorbing alloy having the composition ofMg_(1.9)Al_(0.1)Ni_(1.05) were mixed carbon powder andpolytetrafluoroethylene in a various ratio, and each of the resultantmixtures was rolled, thereby forming a sheet. This sheet was adheredunder pressure to Ni wire nettings, thereby preparing nine kinds ofhydrogen electrodes (negative electrodes). The hydrophobic nature aswell as the elution rate of ions of the hydrogen electrode can bealtered depending on the composition thereof such as the content ofpolytetrafluoroethylene and on the magnitude of pressure for forming thecomposite sheet with Ni wire netting.

The hydrogen electrodes thus obtained were dipped into a 8N aqueoussolution of potassium hydroxide, and then, a charge/discharge cycle testwas performed at the normal temperature. In this charge/discharge cycletest, the charging was conducted under the condition of 100 mA per 1 gof hydrogen-absorbing alloy for 10 hours, and the discharge wasconducted under the condition of 200 mA per 1 g of hydrogen-absorbingalloy until the voltage against the mercury oxide electrode was risen upto −0.5 V. The relationship of the ratio between the capacity at 20thcycle and the capacity at 3rd cycle to the ion elution rate is shown inTable 35. The ion elution rate was determined by dipping the hydrogenelectrode in an aqueous solution of alkali hydroxide for 5 hours, andthen, measuring the amount of the eluted component by means of ICP(Inductively Coupled Plasma) spectrometry. The amount of the aqueoussolution of alkali hydroxide was selected to be about 100 mL per 1 g ofthe alloy in the hydrogen electrode.

TABLE 35 Ratio (%) Composition of of alkali capacity: hydroxide LiquidEluting 20th Ionic aqueous Temp. rate mg/kg cycle/3rd species solution °C. alloy/hr. cycle Example 327 Mg 8N KOH 25 0.3 70 Example 328 Mg 6N KOH25 0.5 65 Example 329 Mg 7N KOH 25 0.4 77 1N LiOH Example 330 Mg 9N KOH60 1.2 75 Example 331 Mg + Al 8N KOH 25 1.0 72 Example 332 Mg + Al 8NKOH 60 3.4 68 Comparative Mg 8N KOH 25 0.7 32 Example 25 Comparative Mg9N KOH 60 2.3 43 Example 26 Comparative Mg + Al 8N KOH 25 4.8 30 Example27

EXAMPLES 333-341

To the hydrogen-absorbing alloy having the composition shown in Table 36were mixed carbon powder and polytetrafluoroethylene in a various ratio,and each of the resultant mixtures was rolled, thereby forming a sheet.This sheet was adhered under pressure to Ni wire nettings, therebypreparing eight kinds of hydrogen electrodes (negative electrodes).

The hydrogen electrodes thus obtained were dipped into a 8N aqueoussolution of potassium hydroxide, and then, a charge/discharge cycle testwas performed at the normal temperature and in the same manner asexplained in Example 327. The relationship of the ratio between thecapacity at 20th cycle and the capacity at 3rd cycle to the ion elutionrate as determined in the same manner as explained in Example 327 isshown in Table 36 below.

TABLE 36 Hydrogen-absorbing alloy Ionic species Example 333Mg₂Ni_(1.125) Mg Example 334 Mg₂Co_(1.1)In_(0.11) All elements Example335 MgNi_(1.11)Ag_(0.22) Mg Example 336 Mg_(1.8)Al_(0.3)Ni_(0.9)Pd_(0.3)Mg Example 337 Mg_(1.8)Al_(0.3)Ni_(0.9)Pd_(0.3) All elements Example 338Mg_(1.6)Al_(0.3)NiMn_(0.2) Mg Example 339Mg_(1.6)Al_(0.3)Ni_(0.7)Mn_(0.2)Co_(0.2) All elements Example 340Mg_(1.8)Al_(0.3)Ni_(0.9)Pd_(0.3) (Powder Mg dipped in 0.01N hydrochloricacid for 30 seconds as used) Example 341Mg_(1.8)Al_(0.2)Ni_(0.95)Pt_(0.05) Mg Composition Liquid Eluting ofaqueous Temp. rate mg/kg Ratio (%); 20th solution ° C. alloy/hrcycle/3rd cycle Example 333 8N KOH 25 0.2 65 Example 334 8N KOH 25 2.172 Example 335 8N KOH 25 0.2 76 Example 336 8N KOH 25 0.2 78 Example 3378N KOH 60 4.2 77 Example 338 7N KOH 60 0.5 70 1N LiOH Example 339 9N KOH60 3.4 80 Example 340 8N KOH 25 0.2 76 Example 341 8N KOH 25 0.2 74

As seen from these Tables 35 and 36, a simulated battery provided with ahydrogen electrode comprising a hydrogen-absorbing alloy which isfeatured in that, when the negative electrode is immersed in a 6 to 8Naqueous solution of an alkali hydroxide, (a) either the elution rate ofmagnesium ion into the aqueous solution of alkali hydroxide of normaltemperature is not more than 0.5 mg/kg alloy/hr, or the elution rate ofmagnesium ion into the aqueous solution of alkali hydroxide of 60° C. isnot more than 4 mg/kg alloy/hr, and (b) either the elution rate of acomponent element of alloy into the aqueous solution of alkali hydroxideof normal temperature is not more than 1.5 mg/kg alloy/hr, ore theelusion rate of a component element of alloy into the aqueous solutionof alkali hydroxide of 60° C. is not more than 20 mg/kg alloy/hr(Examples 327 to 341) had a higher capacity as compared with a simulatedbattery provided with a hydrogen electrodes (Comparative Examples 25 to27) comprising a hydrogen-absorbing alloy which did not satisfy suchconditions as mentioned above.

EXAMPLES 342-348, AND COMPARATIVE EXAMPLE 28

Hydrogen electrodes (negative electrodes) obtained in Examples 327, 333to 335 and 338 to 340, and Comparative Example 27 were respectivelysuperimposed on a paste type Ni electrode (a positive electrode) with anylon non-woven fabric interposed therebetween. Then, the resultantcomposite was wound into a cylindrical body thereby preparing eightkinds of electrode group. These electrode group were inserted into abattery case of AA size, and then a 8N potassium hydroxide was pouredtherein. Thereafter, the case was sealed with a sealing plate providedwith a safety valve thereby obtaining 8 kinds of AA type nickel-hydrogensecondary battery.

The batteries thus obtained were subjected to a charge/discharge cycletest performed under the following conditions i.e., the charging wasconducted under the condition of 50 mA per 1 g of hydrogen-absorbingalloy for 10 hours, and the discharge was conducted under the conditionof 20 mA per 1 g of hydrogen-absorbing alloy until the voltage againstthe mercury oxide electrode was lowered down to 0.9 V. The cycle wasrepeated, and the capacity at 3rd cycle was compared with the capacityat 20th cycle. Further, the batteries were disintegrated at 30th dayafter the manufacture thereof, and the amount of Mg ion eluted into theelectrolyte was measured by means of ICP (Inductively Coupled Plasma)spectrometry. The results are shown in Table 37.

TABLE 37 Ratio (%); 20th Ex. No. of hydrogen Ionic Concentration cycle/electrode used species mg/l 3rd cycle Example 342 Example 327 Mg 1.2 68Example 343 Example 333 Mg 1.1 63 Example 344 Example 334 Mg 1.7 57Example 345 Example 335 Mg 1.2 62 Example 346 Example 338 Mg 1.3 59Example 347 Example 339 Mg 1.2 66 Example 348 Example 340 Mg 1.4 78Comparative Comparative Mg 2.8 34 Example 28 Example 27

As will be apparent from Table 37, the batteries of Examples 343 to 349satisfying the condition that a magnesium ion concentration in thealkali electrolyte 30 days after filling and sealing the alkalielectrolyte in the case was not more than 2.2 mg/liter had a highercapacity as compared with the battery of Comparative Example 28 whichdid not satisfy such condition as mentioned above.

As explained above, the hydrogen-absorbing alloy of this invention isnot only featured as being light and of high capacity, but also featuredin that since it is excellent in low temperature hydrogen-absorbingproperty and chemical stability, the applicability of this alloy can beextended, beyond those to which the conventional alloys have beenapplicable, to various fields (such as the storage and transportation ofhydrogen, the storage and transportation of heat, a heat-mechanicalenergy exchange, the separation and refining of hydrogen, the separationof hydrogen isotope, a battery containing hydrogen as an activematerial, a catalyst for synthetic chemistry and heat sensor). It isalso possible to develop a new field of application making the most ofthe hydrogen-absorbing alloy.

Further, according to the hydrogen-absorbing alloy and surface-modifyingmethod of this invention, it is possible to easily activate the alloyand to improve the hydrogen-absorbing property. Accordingly, with theemployment of such an alloy as a negative electrode, it is possible toachieve a battery of high capacity.

Moreover, according to the negative electrode and alkali secondarybattery as proposed by this invention, it has become possible to apply aMg-containing hydrogen-absorbing alloy to a charge/discharge reaction,which the conventional Mg-containing hydrogen-absorbing alloy has failedto realized up to date. Furthermore, it is possible to keep thestability of the charge/discharge reaction for a long period of time,while retaining a high capacity.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, representative devices, andillustrated examples shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A hydrogen-absorbing alloy, which comprises: ahydrogen-absorbing alloy powder having a composition represented by thefollowing formula (IV): Mg_(2−x)M2_(x)M1_(y)  (IV) wherein M2 is atleast one element selected from the group consisting of B, Be, Y, Pd,Ti, Zr, Hf, Th, V, Nb, Ta, Pa, and Al, M1 is Ni, x is defined as0≦x≦1.0, and y is defined as 0.5<y≦2.5; and 0.01 to 50% by volume of atleast one powdered additive having an average diameter of from ≧0.001 μmto ≦0.8 μm, wherein said at least one powdered additive is dispersed insaid hydrogen-absorbing alloy powder and is selected from the groupconsisting of the following (a), (b) and (c): (a) at least one elementselected from the group consisting of Group Lk elements, Group IIAelements, Group IIIA elements, Group IVA elements, Group VA elements,Group VIA elements, Group VIIA elements, Group VIIIA elements, Group IBelements, Group IIB elements, Group IIIB elements, Group IVB elements,Group VB elements and Group VIB elements; (b) an alloy formed of anycombination of the elements defined in (a); and (c) an oxide of any ofthe elements defined in (a).
 2. The hydrogen-absorbing alloy accordingto claim 1, wherein said (a) is at least one element selected from GroupIIIA elements, Group IVA elements, Group VA elements, Group VIAelements, Group VIIA elements, Group VIIIA elements, Group IB elements,Group IIB elements and Group IIIB elements.
 3. The hydrogen-absorbingalloy according to claim 1, wherein said(a) is at least one elementselected from the group consisting of V, Nb, Ta, Cr, Mo, W, Mn, Fe, Ru,Co, Rh, Ir, Pd, Ni, Pt, Cu, Ag and Au.
 4. The hydrogen-absorbing alloyaccording to claim 1, wherein the symbol x satisfies a relationship0.05≦x≦0.5, and y is defined as 1.05≦y≦1.5.
 5. The hydrogen-absorbingalloy according to claim 1, wherein the composition of thehydrogen-absorbing alloy powder is Mg₂Ni and said at least one powderedadditive is Ni powder.
 6. An alkali secondary battery comprising anegative electrode comprising a hydrogen-absorbing alloy, saidhydrogen-absorbing alloy comprising: a hydrogen-absorbing alloy powderhaving a composition represented by the following formula (IV):Mg_(2−x)M2_(x)M1_(y)  (IV) wherein M2 is at least one element selectedfrom the group consisting of B, Be, Y, Pd, Ti, Zr, Hf, Th, V, Nb, Ta,Pa, and Al, M1 is Ni, x is defined as 0≦x≦1.0, and y is defined as0.5<y≦2.5; and 0.01 to 50% by volume of at least one powdered additivehaving an average diameter of from ≧0.001 μm to ≦0.8 μm; wherein said atleast one powdered additive is dispersed in said hydrogen-absorbingalloy powder and is selected from the group consisting of the following(a), (b) and (c): (a) at least one element selected from the groupconsisting of Group IA elements, Group IIA elements, Group IIIAelements, Group IVA elements, Group VA elements, Group VIA elements,Group VIIA elements, Group VIIIA elements, Group IB elements, Group IIBelements, Group IIIB elements, Group IVB elements, Group VB elements andGroup VIB elements; (b) an alloy formed of any combination of theelements defined in (a); and (c) an oxide of any of the elements definedin (a).
 7. The secondary battery according to claim 6, wherein said (a)is at least one element selected from the group consisting of Group IIIAelements, Group IVA elements, Group VA elements, Group VIA elements,Group VIIA elements, Group VIIIA elements, Group IB elements, Group IIBelements and Group IIIB elements.
 8. The secondary battery according toclaim 6, wherein said (a) is at least one element selected from thegroup consisting of V, Nb, Ta, Cr, Mo, W, Mn, Fe, Ru, Co, Rh, Ir, Pd,Ni, Pt, Cu, Ag and Au.